Ok, when it comes to young Malawi cichlids, sexing them can prove difficult. Most are not sexable until the males begins to mature and develop his adult male colouration.
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