The endo will ask you questions regarding your health and your family's health history. You should expect to have an exterior genetalia exam, where you'll take off your pants, lay on a table, and he/she will examine your penis and testicles (I had this done and my endo was a woman). You will also be asked to remove your shirt for an examination of your chest. Blood will probably be drawn for lab testing.
You should know that gynecomastia can exist because of a low ratio of estradiol (estrogen) to testosterone, which may be caused by a number of things (namely your testes or your pituitary gland). By checking your LH, FSH, SHBG, Testosterone Serum, and Estradiol (perhaps more, but these should be pretty much what you may need initially) your endocrinologist can begin to assess what your situation is. Do not accept results like "normal." Ask for a copy of your quantitative results and have the endo explain what each of them means in regards to your body.
The pituitary gland sends a signal to your testes to produce testosterone, and thus a negative signal is sent back to the pituitary. A low testosterone count with a high LH (the signal back to the pituitary) and high estradiol means that there may be a problem with your testes. If your testosterone is low and your LH is also low, there may be a problem with your pituitary gland.
Testosterone deficieny has a number of symptoms including low sex drive, loss of energy, loss of hair growth (facial / body hair), inability to gain muscle / mass, loss of concentration, and underdeveloped genetalia, among other things.
It's late and I'm tired so if any of this is incorrect, anybody may feel free to correct me. If I can think of anything else I'll post again.