How can I tell if I have gynecomastia or if I'm just fat?

Lose weight... ...If you still have breasts it's gynecomastia. Or pinch it: if texture is dense and cobblestoney is gland if soft and compressible, it's fat. Compare the feel to an abdominal fat roll. Or you can get an MRI or ct.
Firm breast tissue. Actual breast tissue feels firm. Fat is softer. Visit gynecomastia.Org to find an experienced plastic surgeon in your area.