When buying fresh seafood, use your senses of sight, smell, touch.
Look at the seafood to see if there are any spots, discoloring, or brown edges. Don't buy it if it has any of these. They are a sign of aging.
Smell. Simply take a whiff of the seafood. If it has an unpleasant odor, or strong fishy smell - don't buy it. It's been sitting around too long.
Touch. Gently poke the seafood (fish). If the flesh quickly bounces back, it is fresh. If the shellfish has a soft spot when touched - don't buy.
Using your senses when buying fresh seafood will only help you make a healthy, good buying decision.