Unfortunatly, sexing of red bellies is extremly difficult. Females may be a tad more round-shaped, unless they are breeding there is no way to clearly identify them. Breeding of red bellies wont happen, it is near impossible. But the lucky few (and I mean few) that have bred red bellies succesfully, have noticed that they are some prolific breeders!
Perhaps a picture may help identify more clearly? Even with a picture its a long shot, but I cold print the picture out and give it to my friend who is pretty good at sexing piranhas, though red bellies are difficult like Ive said before.