Yes, in most cases, a dentist can tell if a person is bulimic. The stomach acids that come out when a person vomits scars and damages the back of the teeth. This erosion of tooth enamel, along with redness around the lips can indicate bulimia. Erosion of tooth enamel can also change the shape, size, and color of the person's teeth which can also suggest bulimia.
Also, the dental patient might mention experiencing sensitivity and dry mouth. A dentist might also notice a sudden weight loss in a long-term patient that's another indication of an eating disorder. Bulimia is one of the few health conditions that cause such damage to the mouth and teeth. Dentists are trained to spot these signs of bulimia in patients even if they aren't apparent to others.