Salmon is full of protein and Omega-3 fatty acids, and is lower in fat than most red meats. According to the Center for Science in the Public Interest, salmon is one of the best foods you can eat. However, you need to be careful when you purchase salmon, because old fish not only tastes bad but can cause illness. Fortunately, there are tell-tale signs that can help you determine the freshness of the salmon before you buy it.
Smell the fish. A mild smell of the sea is normal, but fresh salmon should not have a strong fishy smell.
Examine the flesh of the fish. The flesh should be firm and have a shiny appearance. If the flesh is dull, falling apart or appears to be mushy, then it isn't fresh.
Rub the scales on the fish. The scales will rub off easily if the fish isn't fresh.
Look at the color of the gills--located just behind the head--if it's a whole fish. The gills should have a deep red or pink color. If the gills have been cut out or have a pale color, you're better off looking elsewhere.
Look at the eyes if it's a whole fish. The eyes should be bright and clear. If they are cloudy and sunken in, the salmon is not fresh.