Best bet is to wait until they grow up. Females will be about half the size of males. Males will be about eight cm long. Males will also be vibrantly colored while the females will be a softer yellow. Females have a very clearly black line running horizontally across the length of their bodies, but I think the young ones do as well, so it's best to wait and see to be sure.
That's what my research suggested anyway. I could have buggered the whole thing.