if it's gyno flare up then, like the guy said already, there will be a hard lump there, feeling like a bead or a frozen pea or something, initially. if you have fat on your chest then it will be hard to differentiate, however, fat won't creep up on you in your chest quickly so if you notice that going on steroids causes a new, or enlarged hard piece of tissue behind your nipple (it will always be behind the nipple) then it's gyno.
letrozole stops and reverts most gyno, cabergoline/pramipexole will solve the other kind that letro doesn't.