when they're starting to flower.... if what u see looks like little hairs sticking out, it's a girl if u see what looks like tiny grapes, it's a male *the hairs should start at the joining part off the stem right above where it meets the leaf* http://www.hightimes.com/ht/grow/
Here's a couple of pics to help you out next time. Remember, boys have balls, girls have fuzz! You want to look in the crotch of the branches for the male pre-flowers. AS SOON AS YOU'RE SURE of the males, yank 'em.
dangggg ive been growin a male as well. oh well it was my spring queen anyways now that summers commin i'll pay more attention