If they're the true Heros severus (mouthbrooding type) then the only difference is a few brownish spots on the gill plates of the males. Try watching their behaviour, if a pair forms they'll hang about together and keep the others at a distance if possible. Good luck with these, very interesting when breeding!
I have had pretty good luck sexing simply off their facial markings once they are somewhat mature. The males tend to have longer and more prominent facial "worm" markings, and once they get larger tend to have a broader chest.
The tell tale signs of pairing are the typical cichlid courting. Lots of "touching", much wiggling in front of each other, mine love to lip lock for hours, and then it moves onto more wiggling and surface cleaning of their selected spot.