How do you know if a hermit crab is dead or beginning to molt?

I'd say he is probably dead. The best way to find out would be to just pick him up and try to see if you can get him to move at all. Usually molting only lasts 24 hours if even that and they're back to normal. I'd try to pick him up and see if I can't get him to move. They're not paralyzed when they molt and if you can't get it to move at all then it's dead. The smell is another sign. Plus when you pick him up to see if you can get any movement/reaction out of him check and see if he is cold all over.