Somewhere I read that female corys seem to be resting on the stomach more than males, almost like they are "fatter" than the males. It won't be that obvious on gravel, maybe put something flat in so they can rest on it. I guess it comes down to the males rest flat, and the female's stomach keeping her off the ground. That's how I sexed mine. Also, the males tend to be smaller...
I will see if I can get some pics...