There's no way to tell by looks alone whether a person is bulimic. Bulimia nervosa, also known as binge-purge disorder, is, like its related eating disorder anorexia, marked by severely restrictive eating, but the extreme dieting is punctuated by bouts of out-of-control binge-eating.
Most often, they are followed by purging, or self-induced vomiting, less often by the use of laxatives or diuretics or other compensatory behavior such as fasting or overexercising.
The disorder typically begins during adolescence, and cognitive-behavioral psychotherapy is the treatment of choice.