How can my doctor diagnose gynecomastia?

Examination. The diagnosis can usually be made by physical examination of the breasts. Sometimes mammography and ultrasound are helpful. Occasionally, biopsy is necessary.
Exam. Visual enlargement of the breasts constitutes gynecomastia. If they're enlarged the diagnosis is obvious.
Yes and more. Diagnosis of gynecomastia is only the first step. Identifying the cause is the next very important one. An experienced primary care physician will be able to conduct a complete workup and direct the next step in care.