If it smells fishy smell another. The fish should have a fresh, yet mild briny smell to it. Appearance is moist and the flesh should have elasticity when poked. It should spring back immediately. The flesh will easily slip away from the bone on an aged fish.
Whole fish is the easiest way to detect freshness.
Eyes are clear, not cloudy, pink or sunken. The gills should be bright red or pink. There should be no slimy feel to the skin. Fish is cold. Preferably packed on ice.