One method to sex african cichlids is by venting them. In other words, looking at their underside to determine if their vent ressembles that of a female oO or that of a male oo.
This article is dedicated to venting information along with some links and helpful photos to aid in your venting efforts.
http://www.fishhead.com/articles/ventsex.htm
http://www.thecichlidgallery.com/article_venting.htm
http://www.cichlid-forum.com/articles/gender.php
Here are some pics of some common Malawis:
Female 1 1/2″ red zebra
Female 1 3/4″ yellow lab
Female 3″ ob peacock
Female 1 3/4″ albino socolofi
Female 2 3/4″ yellow lab (she had spawned earlier that day so the vent is slightly larger than normal)
Female 3 3/4″ kenyi
Male 1″ ob peacock
Same male ob peacock at 1 3/4″
Male 2″ albino socolofi
Male 2 1/2″ socolofi
Male 1 3/4″ yellow lab
Male 4″ auratus
Male 1 3/4″ red zebra (actually, I’m only about 90% sure this guy is male)
Big Thanks to chelle/tmg68650 for the spectacular photos