Place the mussels in a colander in the sink and run water over them, using your hands or a clean scrubbing brush to rub off any debris like seaweed, sand, barnacles, or mud spots that could be on the shell. If you find any mussels with open shells, lightly tap that mussel against the side of the sink. If the mussel closes up again in response to this turmoil, it's alive. If it doesn't move, discard it.
The "beard" of a mussel is the clump of hair-like fibres that sprouts from the shell. To remove the beard from the mussels, grab it with your thumb and forefinger and tug it toward the hinge of the mussel shell. You can also use a knife to gently scrape away the beard.