Instructions. Check the color of the crab. If it is already orange or red, it is cooked and just needs to be reheated. If it is greenish or brown, you'll want to make sure that you steam or boil it until it turns orange or red. Reheat the crab for the appropriate length of time.now crabs and King crabs have large meaty legs that are removed from the crab on the boat, cooked, and flash frozen to enable them to be transported and stored without degrading in quality. So, if you see crab legs in the fish market, they are probably already cooked.