Well, to check the gender of your fish you can look for a gonopodioum, it is like a small stick where the anal fin it. This is what the male has, the females have nothing there. Also, your platies may well be breeding, but the fry are just being eaten, as platties can eat their fry alot of the time. To save them you will most likely need either another tank or a net breder. (Do not get plastic breeder traps, they stress the fish) And dont put the fish in the net breeder if you get one, wait for it to drop babies and then put the babies in there.