Raw: open = bad, closed = good.- Cooked: open = good, closed = bad
If you buy mussels in from a fishmonger or in a supermarket, they ought to come cleaned with the beards removed and they should all be tightly shut. If there are any open ones, give them a tap on the counter. If they immediately shut, that’s fine, but if they stay open, the mussel inside is dead, and should be thrown away as, most likely, it will make you sick.
If you get mussels that still have the beards attached, (they’re the obvious stringy tangles on the shells) simply tug them off and give the shells a good scrub.
If you’ve bought mussels on the pier from some guy selling them out of a bucket , it’s a good idea to rinse them by dunking them in a bowl of cold water and a spoonful of flour for ten minutes. The mussels will eat the flour and expel any sand, cleaning themselves.