Well i will start off by saying that it was a mistake to plant all those seeds in one pot like that, itll be a bitch tryn to re pot them all, so you will probably loose a few. And They are way to young to tell, so re pot, then wait until youre done veg cycle. Then 2wks into flowering you should see lil white hairs, or banana looking clusters of balls. So in closing; Females will have what are called Pistils. and that's what you want. If you see Balls then get rid of them.
This Lady was born March 30 2010, and it is now may 15, and i just det. sex on the 12th, i chopped the male that was growing with her that same day!! This lady is in her first wk of flowering. I am using organic soil added Cow Manure (Composted). She has about 7 weeks till harvest. So yea it takes about a good 7 weeks after seedling to Determine sex.