Smell. By far, this is the best way to determine freshness. Fresh fish should have a slight sea or salty smell to it or no odor at all. Any off-odors or ammonia smell are a sure sign of age or poor handling.
Texture. Generally, the texture of fish should be firm and spring back when you press against it with your finger.
Appearance. Fish fillets should be moist and glistening.
Check the eyes. If purchasing a whole fish, the eyes should appear clear and full. Over time, the eyes will begin to lose moisture and sink back.
Check the gills. If purchasing a whole fish, the gills should be pink in color with no traces of brown or grey.
Check the belly. There should be no sign of "belly burn", or rib bones protruding into the belly cavity, which indicates that the inside of the fish was not removed promptly. There should be no tearing or breaks in the meat.