how do you know if you have breast cancer?

-by doing a self breast exam, how would you know that you have breast cancer?
-at the end of my nipples, it’s red (sometimes) and only sometimes, the skin will peel a little bit–not that much!! i don’t feel any bumps or lumps, but i’m a little worried about the nipples.
-i’m not sure, but i used to wear a bra with stitching on the inside of the cup, and it could be from that, since it scratched it.