Scallops that have gone bad generally are slimy and dull in color. In addition, this typically sweet-smelling seafood may develop a strong fishy odor which is not a trademark of fresh scallops.  If you notice any of the aforementioned signs before consuming your scallops, you may wish to toss them and purchase new scallops fresh.  Remember, consuming any food that is old or expired may cause illness or food poisoning symptoms. Never leave scallops out at room temperature for longer than 2 hours and always refrigerate scallops for no more than 2-4 days.