Your nose will know. The market where you shop for seafood should smell sweet and briny, like the ocean. If it smells like fish, go to another market.
Look closely at a fresh whole fish. If its eyes are shiny and its gills are deep pink, the fish is fresh. If its eyes are dull and sunken and its gills are pale, choose another.
Cut fish—whether cut into steaks, fillets, or halves—should be displayed on ice, never in pans or on platters. Look for moist flesh with a slight sheen. Beware of fish that look dried out.
When purchasing crustaceans such as shrimp or lobster, ask the fishmonger to scoop up a few so you can give them a good whiff. They should smell like a sea breeze.
It is difficult to evaluate the freshness of mollusks, such as clams and mussels, because their flesh is enclosed within their shells. While cooking clams, discard any clams with shells that do not open. When purchasing mussels, tap lightly on the shells—if the shell snaps closed, that means the mussel is alive and fresh enough to enjoy.