How do you determine the sex of a hermit crab?

Generally size is a good indication, although that's not always the case. Unless you got it from a pet store that it quite familiar with the background of the animal, you're best bet is probably to judge from the size. THis link has detailed information of the ages of hermit crabs and how to tell: http://crabstreetjournal.com/xoops/modules/news/article.php?storyid=16