Firstly, if you wouldn't mind posting some pics, many of us here are experienced with what sort of shapes gynecomastia comes in, and so we may be able to tell just from simply looking at some side-view pics of your chest.
Also, rub around you areola and breast area (push kind of hard) looking for any lumps or general mass that doesn't exactly feel like fat. It may not necessarily be a bump directly under your nipple but by rubbing somewhat hard you should be able to feel some lumpy mass that you will know shouldn't be there.
Please post pics (crop out your face if you like). Nobody is here to judge, just to help.