I have spent literally hours trying to figure this thing out, and the best conclusions I've come to are that there's a random breeder somewhere in Wisconsin who figured out that right before lights out, the females get slightly yellow in coloration and their bellies distend slightly. That's a pretty tough thing to notice, however.
If you can figure it out, or if you have other behavioral characteristics to go by (the males are more agressive, for example), you can more than pair them, you can actuall harem them. One male will go with two females.
Purple firefish are a little more agressive, but the same thing applies if you're lucky.
This is just from some of the papers I've read on the matter, and from previous posts here.