You can not tell the difference between male and female. Some people will argue that the lump on the head tells the sex but IMO
not the case. That is between two fish. Just like how someone said only male kribensis have spots on their tales. Well two of my three females have spots as well. The same has been done with clown fish.. I believe around 4 to 5 I inches they will breed if you have a pair. If you have 2 there is a 50% chance you have a pair 3 is 75%, 4 is 88%, 5 is 94% and 6 is 97%.
Now do keep in mind, If you have two females they could both let eggs at the same time. And if there is no male, nothing will fertilize the eggs.