How will my doctor be able to tell if I have bulimia nervosa?

My evaluating you. Your doctor can diagnose you with bulimia based on your answers to some questions about your eating habits, the frequency and duration of the habits, your body image, and other presence or absence of any medical/physical and psychological symptoms.
Interview. The diagnosis is made based on a particular set of symptoms and can only be diagnosed as a part of a clinical interview with your doctor.

Related Questions

How can my doctor be able to tell I have bulimia nervosa?

By what you say. Your description of your eating and purging behavior is the primary way anyone who doesn't know you very well can tell. Read more...