It's hard to say. Generally we find out when they are not happy because they behave as if they are ill, moving slowly, hanging out of their shell, not moving for many days.
A hermit crab who is healthy will probably be seen walking around in the tank, climbing on fake plants and wood, or digging in the substrate. He will visit the water bowl regularly, visit the salt water bowl occasionally, and will generally trash the tank sometimes, toppling dishes and dragging bits of fresh food around.
If they disappear for weeks, they are probably below the surface molting, and should never be disturbed at this time. Never, ever dig up a buried crab.
Be sure to visit http://www.crabstreetjournal.com and look up the article "Are you Ready for a Land Hermit Crab?" to make sure you have everything he needs to live a long, healthy life.
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