How do you know if a mussel is bad?
They should not smell overly fishy. The shells should be closed tightly. If you find any mussels in your bag with open shells, gently tap them on the counter, wait a minute, and see if they close. Discard any mussels don’t close, or those with chipped or cracked shells.
Wash and store the mussels first thing when you arrive at home, if you aren’t going to cook them right away.