(Courtesy of Red Lobster)
The fish's eyes should be clear. If the eyes are cloudy, the fish probably was harvested more than five days ago and is beginning to get old.
The fish's interior gills should be bright red. If the gills are pink or brownish gray, the fish has either been mishandled or is getting old, indicating the quality of the fish may be poor.
There should be no offensive odor. Fresh fish does not smell offensive.
Generally speaking, fresh fish should smell moist and fresh, comparable to a cut cucumber or melon. If there is an unusual odor, the fish is probably old and should not be purchased.
The fish's flesh should be firm. The flesh should be firm to the touch and should "spring back" in place. If an indentation remains after the fish is touched, the fish is beginning to get soft and is probably old.
If you're purchasing fresh fillets or steak cuts and not a whole fresh fish, there are no eyes or gills to examine, but you can check for several other things:
The first is gapping, or the separating of flesh in a fillet that indicates the fish is getting old. The layers of meat should not separate or "gap." A minor amount of gapping is acceptable; however, a large degree of separating and soft flesh is a good indication that the fillet should not be consumed.