|Hairy hermit, Pagurus hirsutiusculus. With anemone.|
The only usual way to tell a female hermit crab from a male is to catch her out of the shell, not moving (which almost never happens) and look for two tiny dots on the underside of her belly. I've managed it once.
The other two signs are:
1. A mature male captures a female. (They seem to have no problem distinguishing which is which.) He grabs her by the edge of her shell and carts her around, sometimes for days, until she is ready to mate. This was happening a few weeks ago.
2. The female is carrying eggs. These will be attached to the side of her thorax, (the red area on the side of the one above.) As they grow, they fill in the gap and get in her way; she pushes them outwards and fans them, jiggles herself around in the shell, trying to get comfortable. At this stage, there's no doubt about her gender.
So: is that red area the beginning of the growth of eggs? Or not? Wait and see.