Lift up their skirts!
Net the fish. With wet hands, gently turn the fish over to look at the bottom. There are two pores just to the front of the anal fin (the unpaired fin on the bottom, or ventral side, of the fish). The pore closest to the head is the anus, and will be similar in both genders. The pore farthest from the head, between the anus and the anal fin, is the genital papilla. Generally speaking, the genital pore will be smaller in diameter in males than females. Here's a good link to illustrate the basic differences, though all the fish illustrated are Lake Malawi mbuna.
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Google pics of sexing or venting Neolamprologus brichardi to get some pics of how this species looks. With a little practice, it's really pretty easy.
WYite
Edit: P.S. IME it's easiest to vent with a bright indirect light that limits both shadows and glare. Ya also wanna keep the fish mostly in the water while doin' this. It seems to limit the stress on the fish, especially when yer new at it and hafta take a few seconds to make sure yer seein' things correctly.
