some plants will show sex during veg.. usually 4 - 6 weeks into veg they will show pre flowers... if you want to know earlier than that you can cut a clone, root it, and put it under 12/12... after about 2 weeks (one week to root and one week under 12/12) you should be able to tell if its male or female... if its male remove the plant it was cut from... if it is a female then you know what plants are females...
males look like they are growing little grape clusters at the nodes....
females have two little white hairs, called pistils, that will pop out at the nodes....
hope this helps..
sampson