3-6 weeks after sprout, the preflowers should either show hairs or bananas. Depends on the strain, so just be patient. I know, it's hard, I'm in the same boat, but got two confirmed females now (out of 5 plants), and no bananas on the other three, so they're likely female too...but I'll wait until I see hairs to be sure before I transplant them to a bigger pot.

There are other ways to determine sex too, like taking a clone and flowering that, or putting a completely opaque black baggy around one stalk to flower just that stalk. Or just flowering your plants, of course, once they have calyxes. But I just wait 'em out, personally.