Peacocks, if they have no color, female (assuming they are of any real size). Yellow labs, no true way except venting or just waiting till a female is holding. Don't know about compressiceps.
So basically haps/peacocks easy to tell. Mbuna, well often the males are different colors than female.
But they have to be of the size where they start to color up or start to change color, and of course that size always depends on things. IE in a tank that a peacock feels safe in I have an ~1 inch guy showing more color than a 4 inch guy in a tank where he is scared.