They are slimy and they smell. Otherwise, they have little odor and tend to take on the ammonia smell the longer they sit. Crabs should only be eaten when you buy them alive and steam them live. Otherwise, you must rely on your seafood place to steam only live crabs. Again, ask around.
Dead crabs deteriorate quickly and smell.
Refrigerated crabs can be kept 2-3 days. Soft Shell Crabs should be alive or frozen when bought. Thawed crabs lose their taste quickly and fresh cleaned crabs are okay, but lose their taste after a few days. Live crabs should be soft to the touch. The softer the better.