The most reliable way to sex a hermit crab is to look for the presence or absence of gonopores. Gonopores are tiny round openings at the base of a hermit crab's last pair of walking legs.
Females will have gonopores.
Males will NOT have gonopores.
To check your crabs, you can wait until they try to climb the glass (like the one in the photo is doing), or you can place them in a glass bowl and try to look at their underside. You can also hold them by the shell and use a flashlight to check their underside if they come out far enough.
NEVER never never try to pull a crab out to see. And be careful of the pinchers!
Often pet stores and other sources will give misinformation when asked how to sex hermit crabs. You cannot use claw size, overall color, number of claws, shell type, etc. Gonopores are the only reliable and accessible way to tell that we know of.