Clams that have gone bad are usually dead and toxins that build inside the dead mollusks like clams can be transferred to you if you cook and eat them. Thus, take a really good look at the clams before buying them, and also inspect them right before cooking them. The entire batch might be toxic if you have to throw out more than three of them.

The primary indicator as to whether or not clams are good is if they are open or closed. Clams should be slightly open and should close rather quickly when tapped, but if the shell is closed, or doesn’t close when tapped, the clam is dead. That's when you need to toss them in the trash.

Another way you can tell if clams are dead if you'd like to double check is to toss them in a bowl of water, and if they float, they are dead. If you have fresh clams stored in the refrigerator, they should last between seven to 10 days, while shelled clams and frozen clams can both last four to five days in the fridge.