I believe it is very difficult to tell, you have to wait until the are pretty large and they have to be vented, but even then its not too easy. I think the only hard and fast way to tell is when you find eggs in the tank haha
well i have a larger one in a dif tank but i wanted to try and breed them so i got a new smaller one but very tiny lol about 1-2" but i wanted to try and get another of the oposite sex but i didnt want to have to buy 15 of them just to get males and females haha