If you suspect you have an eating disorder, it is important that you see a health professional that is qualified to diagnose eating disorders. Ideally, a psychologist, general practitioner, or psychiatrist, that has a special interest in treating eating disorders.
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That said, the Eating Disorder Examination - Questionnaire is a good indicator:
https://insideoutinstitute.org.au/assets/ede-q-eating-disorder-examination-questionnaire-subscales.pdf

Global score can be calculated by finding the average of your scores 1-12 and 19-28 (adding scores 1-12 and 19-29, and dividing that number by 21). Ideally, a healthy relationship with food and the body will lead to a score close to zero out of six. Disordered eating is indicated by scores of one or higher, and scores of three and above indicate a severe and complex eating disorder.

While there are cases of individuals recovering without treatment, this is very rare.  We see individuals with eating disorders of all ages, with many reporting that they too were of the belief that they would “age out” of the disorder. It is also worth noting that given that Anorexia Nervosa is the mental illness with the highest mortality rate, hoping an eating disorder will just go away on its own is a potentially deadly mistake that will involve years of suffering.

To look for a single cause is to misunderstand the nature of eating disorders. Eating disorders are a coping mechanism that arise out of multiple circumstantial pressures as well as a genetic predisposition.

The best evidence we have for the cause of any mental illness is the biopsychosocial model.
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Biological risk factors may include:
- Family history of eating disorders
- Family history of anxiety 
- Family history of mental illness
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Psychological/personality risk factors may include:
- Over-controlled personality type (strong need for control)
- Perfectionistic tendencies 
- Being academically or athletically gifted from a young age (reinforcing the valuing very high standards and entangling achievement with identity and worth)
- Low self-esteem
- Discomfort with sexual maturation 
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Social/environmental risk factors:
- Having a parent with an eating disorder
- Parent/carers or family members that engage in diet culture (either through discussion or behaviours)
- Emphasis on success or achievement in the family home
- Poor boundaries in the family home 
- Having a physically or mentally unwell family member 
- Trauma - all forms (see cPTSD)
- Early or late onset of puberty
- Experiencing starvation syndrome
- Food allergies leading to a distrust in food
- Anecdotally, we have observed that many of our clients with eating disorders participated in dance, gymnastics, swimming, or running during childhood 

Roughly 10% of those diagnosed with an eating disorder identify as male. Interestingly, during our years of work in an inpatient facility, it was rare for us to see cisgendered* men in an inpatient eating disorder program. It is possible that cisgendered men are more likely to go undiagnosed due to the false belief that eating disorders is a “female” mental illness. We also suspect that men with eating disorders are less likely to present for treatment. 
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*Cisgender refers to individuals whose sense of identity and gender aligns with their birth sex. In the case above, it means identifying as male and have being assigned male at birth

Yes. Unfortunately, this is a common misconception of eating disorders. In fact, Christopher Fairburn’s research indicates that less than 5% of people with eating disorders are underweight. So while a low BMI (that is not caused by a physical health condition) is often indicative of an eating disorder, not being underweight does not tell us at all whether someone has an eating disorder.

Yes. While the Diagnostic Statistical Manual requires individuals to be below a certain BMI to meet the criteria for Anorexia Nervosa, it does recognise Atypical Anorexia Nervosa (individuals with AN that do not meet the weight criteria) under its diagnosis of Otherwise Specified Feeding and Eating Disorders (OSFED).
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For those working in the field, this distinction is commonly understood as unhelpful and inaccurate. Unhelpful in that it focuses on weight (not the most reliable indicator for malnourishment as someone with a “healthy” BMI can be severely malnourished), and inaccurate in that ‘atypical’ implies that it is rare, whereas many people with restrictive eating are not underweight (as mentioned above, less than 5% of people with eating disorders are underweight).

Yes. Unfortunately this is not well understood in the general medical community. This is part of why it is so important to have an eating disorder informed treatment team. While being underweight is an indicator of malnourishment, being at a BMI of 20 or above is not and indicator of adequate nourishment. Individuals with restrictive eating behaviours are prone to be low in magnesium, potassium, phosphate, iron, white blood cells etc. This can be regardless of their weight. 
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Research also indicates that antidepressants are less effective in individuals with compromised intake (through restriction, purging, or excessive exercise) regardless of their weight. Researchers suspect that in cases of undernourishment, the body does not have enough surplus energy to be directed towards building new neural pathways in the brain, rendering the medication less effective. 

Restricting food quantity (via volume or calories) is well known as an eating disordered behaviour. However, eating can be restrictive in a number of ways, such as;
- Time eg. Only eating before, after, or during certain time frames
- Food type eg. Avoiding food groups or types, or sticking to a diet
- Compensation. This involves rules around compensation which can take the form of purging, laxatives, exercise etc. 

For those with a BMI below 20
The short answer is yes. The research is clear that those who reach BMI of 20 have a significantly better chance of recovery. Most individuals with eating disorders that are underweight need to be admitted to achieve weight restoration. That said, individuals have managed to do it on their own if they are highly motivated and have a good support team. People that are significantly malnourished must (at least initially) be renourished in an inpatient setting due to the risk of referring syndrome.
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BMI of 20 is the minimum weight to achieve recovery. Some individuals may find that their body has a higher set weight. Individuals that developed an eating disorder in adulthood can used their weight prior to the onset of the eating disorder as a guide.
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For those with a BMI above 20 
Possibly. Most individuals find that during the renourishing process their weight tends to slowly incline up as their body adjusts to the new intake. After a while their weight might slowly start to drop as their metabolism recovers. This period generally takes around 18 months, and it depends on the persons set weight.

When malnourished individuals are renourished, the body releases insulin into the bloodstream which can lead to a drop of the potassium, phosphate, magnesium, calcium, and sodium levels in the blood stream. This rapid shift of fluids and electrolytes can be fatal. It is for this reason that individuals that have been severely restricting their intake for a period of time need to be renourished in an inpatient setting where they can be adequately monitored and slowly and adequately renourished.

Evidence indicates that the body may have a set weight. This is the observed tendency of the human body to gravitate towards maintaining its weight within a preferred range. It is believed that when individuals are above their set weight their metabolism speeds up slightly to compensate, and that when individuals are below their set weight their metabolism slows down. This tendency of the body causes much anguish to dieters and those with eating disorders who are in a fruitless battle to fight their bodies inclination.

Starvation syndrome refers to the psychological and physical effects of prolonged dietary restriction. The best information we have on starvation syndrome comes from the famous 1950 Minnesota Starvation Study by Ancel Keys. The study found that when individuals restricted their intake for a period of 6 months, they exhibited the following symptoms:
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Physical changes
Low energy
Constant fatigue
Early waking
Difficulty getting to sleep
Insomnia
Dizziness
Headaches
Hypersensitivity to noise and light
Hair loss
Poor circulation leading to cold hands and feet
Fainting
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Psychological changes
Feeling flat or numb
Intense mood fluctuations
Pre-occupation with food
Intense negative emotional reactions
Low motivation
Irritability
Low libido
Poor concentration
Inability to make decisions
Impaired problem solving 
Impaired comprehension 
Preference for food that is hot or cold
Sudden interest in food and cooking
Feeling anxious about meal times
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Behaviour Changes
Unusual food routines and rituals
Binge eating
Spending a lot of time cooking, reading, thinking, and talking about food
Preference to eat alone
Urge to feed others
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Social Changes
Feeling more critical of others
Preference for being alone 
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Additionally, the participants basal metabolic rate (their metabolism at rest) dropped by an average of 40%. This observation supports set point theory. 
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Read a detailed handout on the findings of the study: The Effects of Starvation Behaviour (link to pdf)

Anecdotally, we have observed that the longer a person has engaged in restrictive eating behaviours, the quicker their body “adapts” to starvation by their metabolism slowing right down to conserve energy and prevent weight loss.

No. Restrictive eating, like all other eating disorder behaviours can be best described as a compulsion. The individual is compelled to restrict, and even when it is extremely important to them to not restrict (eg. At a wedding, on a date, when eating with their child etc.), they may find themselves unable to do so. In this way, eating has become like a phobia. Unfortunately this phobia is often celebrated in popular culture as “self-control” and so positive feelings towards the behaviour can mask the fact that it is not a choice.
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Similarly, individuals may be compelled to binge, purge, use laxatives etc. The individuals feelings (positive or negative) towards the behaviour does not change the fact that it is a compulsion and therefore incredibly difficult to stop.

No. In fact, individuals with eating disorder generally desperately crave food and think about it much of the time. Furthermore, individuals with eatings disorders are often deeply afraid of how much they like food. This can cause immense shame and guilt, and they may avoid food to avoid these feelings, as well as to avoid feeling out of control and avoid weight gain.

Our position is that the concept of food addiction is a serious misunderstanding of the underlying issues that lead to and maintain and eating disorder. Someone with an eating disorder may be experiencing intense self-hate, distress, and feelings of being out of control, and disordered eating has unwittingly become a coping strategy. Eating disorder behaviours are often self-perpetuating, making them very difficult to resolve. 

YES. In our opinion, recovery means not only developing a healthier relationship with food and the body, but also addressing the underlying issues that led to the development and maintenance of the eating disorder. 
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We have personally witnessed many individuals achieve full recovery.