Girl hamsters have rows of teats on each side of their belly. These can be tricky to see but if you gently hold your hamster like in the picture below, you should be able to get an idea as they create a fleck in the fur. You should also be able to see the lady-bits between the hind legs.
There is no sign of flecking in the belly-fur. You can see the butt and the hammy-manhood (circled in the picture below). The butt is a noticeable and prominent bulge around the tail area. Take a look at the picture below and notice how the female is flat across the hind legs whereas the male has the bulge. You can sometimes tell whether it is a by looking from above this butt bit is so characteristic.
Female is on the left and Male is on the right.