Typically at. 2-3" you can start to tell. You can vent them at anytime. You will need a magnifying glass though until they get bigger. Read up on sexing Aulonocara, I have a pretty good eye. IMO
they are one of the easier fish to sex, but they also have "fool signs" too. Makes will have female traits, and vice versa.
In do time it will be easy to tell.