I have heard that a pretty good indicator is if you try - genuinely, honestly try - to change and find you can't, you probably have an eating disorder. It may not be obvious at first, but after awhile, if it's an eating disorder, you shouldn't have much trouble spotting it. There is also a questionnaire on the OA website (not that I'm suggesting you undertake the program itself, but it is a tool that can help you determine whether you might have an eating disorder):

http://www.oa.org/newcomers/is-oa-for-you/