sex of mollies

This thread may help you out--very similar for mollies.

http://www.aquariacentral.com/forums/showthread.php?s=&threadid=5769

Male mollies have ..erm... tonkers.

This one: is a female - notice the anal fin (the one near the tail on the underside) is triangular.

This one: is a male - note it has no triangular anal fin, but does carry a little boy's equipment.

The other noteable difference is that the secondary sexual characteristics - sailfin dorsals, for example, are carried only by the males.