I don't know how true it is but I have read that the back of the dorsal? fin is more rounded in the female then the male and that females bellies get reddish when they are ready to breed. I just got a group of six and I seem to see these differances, though mine are different sizes ages. I will be interested to see what anyone else has to say.
Rach, they are easiest to destinguish at breeding time. The female will develope a light pinkish patch on her belly and the male will not. Other than that the most reliable way to sex them is to vent them. The dorsal fin method is not always the most reliable way to sex them. The first 3 spines in the dorsal are probably an easier method. On the male they will be taller than they are on the female giving the female a more gradual slope to the front of the dorsal.