You really can't. As colony fish, the best way to breed and keep thesm is to just get a bunch of them and let them go to work. With an appropriate group gender ceases to matter, except that you need both in the tank to breed of course. You'll notice the gender based on size, behaviour, and which ones end up holding. All the other signs (colors, finnage, egg spots) are very unreliable methods of sexing them.
