How do you know...

whether or not you are anorexic?

Just a thought I had...

You talk to your doctor.   Anorexia nervosa is a serious clinical condition and needs to be diagnosed by a professional and treated.  If you suspect your eating behaviour is abnormal or feel that a preoccupation with food and body-image is preventing you from leading a normal life... if you're depressed or anxious.... then you may benefit from talking to someone close in the first instance.