The female land hermit crab has two openings on the under side of her third set of walking legs. If you have a crab that is willing to extend out of its shell far enough, you may be able to see the gonorpores. They are very small pinhole openings. Males do not have the openings. Female land hermit crabs also can have an egg sac on the side of their abdomen. This is hard to see also and can be confused with the molt sac.

Males of Coenobita Clypeatus are distinguished by the presence of tufts of hair concealing openings on the first segment of the last pair of legs and by the absence of appendages on the abdomen. Females have bare openings on the first segment of the third legs (counting the claws as the first pair) and three forked appendages on the left side of the abdomen for this attachment of eggs.

The Caribbean Terrestrial Hermit Crab Coenobita Clypeatus (herbst, 1791) 

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Male hermit crab:

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