Well, you could wait until they're about...oh...three or four months or so....and the ones with the pink hangy things are boys, and the ones without them are probably girls!
Or, you could pull up the "vent" area to see if a penis will protrude, or if it's a vulva. A vulva has more of a "slit" opening, and sort of resemble a penis, but the penis looks like...a penis. If you're lucky the females will be in the "receptive" mood and be a dark pink.
There's also feeling around to see if you can feel the scrotum. That's usually not the best way to do it though.