You can tell the males and females apart simply by their fins in that the upper part of the male dorsal fin is iridescent and becomes yellow toward the back where the female fin is almost completely yellow.
Any truth to this? I have 6 of these guys. 3 of my fish show signs of this(2.5" or so). Well not 'iridescent' so much but kinda clear/white then tapering to all yellow the last 1/3 of the dorsal fin. The smallest of these 3 is definitely dominate over the whole group of acei. I have no doubt he is a male, and am looking forward to seeing him grow up.
Another question: Is the dominate male in mbuna always the largest?