@ddljohn-- You will know when you have gynecomastia because your breasts will become very tender and painful.
I used to think that gynecomastia is very common and normal because many overweight men and elderly men have it. But apparently, it's not normal. And simply the appearance of larger breasts isn't enough to diagnose gynecomastia. Doctors have to run tests like an ultrasonography or a biopsy to diagnose it. Sometimes muscle overgrowth or breast cysts create the appearance of gynecomastia as well.