[MoribundMaverick]

Well my crabs tend to do really WEIRD things with molting, but I can usually tell when they become a little less active, they start digging a lot, and eating a lot.

They usually bury themselves when they molt, but I've also had my share of surface molters.
If you get a surface molter he actually will look like he's dead. They hang half way out of their shells, or sometimes almost all the way, and just lie really still. At first it will smell fishy, but after a few days if the smell doesn't increase, then that means it's most likely molting. This should be confirmed by you slowly seeing their exo come off.

I actually had a crab molt in 12 hours, but that was only ONCE, and I've never heard of any other crab doing that. I've heard that it can last up to four weeks, but it also depends on the size of the crab.

Hope that helps