after a while (like 4-6 weeks in veg), you willl be able to see preflowers, if you look right where the new growth tips are develloping, you will see a little ball, if this little ball has 2 little tiny white hairs coming out of it...its a female, if it doesn't have little white hairs sticking out then it may just not be mature enough to tell it's sex, but if after another week or 2 you see more and more balls appering but no white hairs, then you prolly have a male on your hands, i find that this all happens very quickly once it starts, you may see no balls at all and then 2 days later you see lots of female preflowers... check out the grow faq on this site and it will give you pics...but the pics are pretty poorly explained, the male and female pre-pre flowers look exactly the same... but they will give you some idea.
and as always if your not sure just wait and all will become clear in a few days.
-Cult