If you look at the underside of your crab, on it's last pair of walking legs under it's abdomen, a female will have a dot on each side, these dots are called nonporous. They are what determines whether or not you have a female or a male crab.
its hard to tell if your hermit crab is a male or female when there in their shell the girls are most likely to be smaller in size than the male but never ever pull your hermit crab out of its shell to see if its a male or a female they probably would like to be pulled in half then give up its home