To be sure you choose the freshest fish, use your eyes and your nose. The flesh or skin should be shiny, the eyes clear, and the gills should be nicely pink. Most good fishmongers will let you smell fresh seafood; if it’s old, you’ll be able to tell.
tbear says some fish varieties hold longer than others. Flat fish like sole and turbot are more perishable. Ask when they were caught, since these fish are often caught and brought to market same day.
Large fish, like tuna and swordfish, last longer if they aren’t cleaned immediately. If they’re caught at a distance from the market, they’ll be flash-frozen on the boat–which is not necessarily a bad thing.
Oysters and mussels hold surprising well, because they’re stored alive. Toss out open ones, which may be dead. A test for determining if they’re dead: tap them on the counter. Good ones will close right up!
Most important: get to know your fishmonger.
Board Links: Buying fish on a Monday at a supermarket