Get a pair of long hemostats or long tongs. Wrap the tip with duct tape (well, anything to soften the tip) and pick up the rat with the tongs by the tail. You should be able to see their bottoms then. If the 2 holes on their bottoms are far apart, then they're males. If they're close together, they're females. If the rat is big enough, you can also see their scrotum bulging out - but sometimes when they're hanging by their tail, the scrotum lays flat to their bodies so it's hard to tell from that. Anyway, there are pictures up there somewhere on one of the stickies...