i was thinking the same thing. i have a 14" one i was going to sell but decided to keep. i *think* he/she was digging in my gravel a few times to make a nest, but i am NOT 100% sure on that. i think that would be the best way to sex a male, nest building. i have bred both red bellies and gold piranhas (spilos/macs) and that is what the male does when he gets ready to breed. it only happened a few times though, so it could just be easily another behavior for him/her.
i've heard of people trying it before and never have it working. one would have to have an enormous tank, i would think, and even then i think there is a good chance of one killing the other.
i havn't seen that, though, before, in several years of only raising piranhas...laying eggs.
i am tempted to raise the temp in my tank, just to see if the one i have starts nest building.
and even then, with both eggs and a willing male, it would probably still be a real long shot.
the only p's i have heard of being bred are red bellies, golds (not too hard for those two species), and caribe and ternetzi very very rarely.