I hate to be sarcastic...but...you check for nipples when you sex rats?

I check for giant testes hanging under their tails. LOL

Seriously, of course male rats have nipples, every guy I know has nipples. If you are having problems sexing your rats, pick them up by the tail and let them grab on to the edge of their tub or tank. Then lower their rear ends below their heads like they are "hanging" off the edge by their front paws. If it is a male the testes will "fall" out and be visable to you from underneath.

Or if you look at animals that you know are males lift them up and look at the distance between the anus and where they urinate. It will be at least twice the distance than in females.

Good Luck!