i don't think there is any reliable way to distinguish males from females.......are you interested in breeding them? i know they're known to do so in captivity....but i doubt if anyone's ever had any luck with rearing babies.....but you can always try.....maybe you can put a bunch of valentini....i say...at least 6 to ensure that you have at least one female or vice versa.......in a large tank....supposedly something like 120.....i'm not expert at breeding fishes...so you might want to sak someone with the actual breeding experience.....