[Jeir]

If you put the hamster in a clear glass and hold it on its side (obviously blocking the open end so the hammy doesn't jump out), you can lift it up so you can see your hamsters' undersides. A male would have an obvious rounded rump and two openings that are far apart, while a female would have a flatter rump, two openings that are very close together, and two lines of nipples on either side of the tummy. There are plenty of visual guides on the 'net to help with sexing if you need them.