Molting at the surface
Since you are assuming until proven otherwise that your hermit crab will molt if it appears lifeless and is in an isolation tank, leaves it alone and watch what happens. If your hermit crab is in the main tank with other hermit crabs, especially if they are on the surface, cut off the ends of a two-liter bottle and sink it into the sand to surround the crab with a clear protective barrier.
Do not disturb a crab that is hanging limply out of its shell, but protect it from other crabs. When they molt, they should continue this process if given time. Once they have died, they will start to smell bad within a few days. A hermit crab can take up to two months to complete the molting process. So you will know long before this time has passed whether it is still alive or not; smaller crabs do not take as long to complete the molting process.
If you find what appears to be a dead crab on the surface next to an empty shell, take a closer look to see if it is just an exoskeleton. If it is hollow and crumbles easily, it is an old exoskeleton and your hermit crab has already molted its shell. Take a quick look in a nearby shell and you may find your molted crab in its new home.