This list is for those of us who are to lazy to cut, gut and fillet our own fish. It comes from the book Fish and Shellfish by James Peterson, a book I highly recommend.
What to look for when buying fish steaks or fillets:
Sheen: The surface of the steaks or fillets should have a moist sheen with no hint of a film or slime layer.
color: The coloration on white or pale steaks or fillets should be cherry red and not brown. This coloration is especially noticeable on swordfish or shark steaks. Irregular red coloring on white fillets or steaks may indicate brusing if the fish was mishandled when caught.
translucence: Fish fillets and steaks should appear slightly translucent. Fish that looks opaque may not be fresh or may have been frozen improperly.
handling: Tuna should not be sliced into steaks but should be kept in as large a piece as possible. Tuna should be wrapped tightly in plastic wrap and should be translucent pink or red with no hint of brown. opalescence: Some fish, especially tuna, may develop a rainbowlike opalescence on their surface when handled improperly. While some oily fish take on this opalescence even when in perfect condition, it’s best to play it safe and avoid buying fish with this appearance.
grain: Don’t buy fillets or steaks with spaces in the flakes. All fish should be dense with no gaping visible anywhere on the steak or fillet.
packaging: If you buy fish in those little cellophane pachages popular in supermarkets, make sure the container contains no liquid.
buying tricks: Avoid buying what appears to be the last pieces of a large fillet — such as a salmon tail piece — or the last slice of a tuna or swordfish loin. Tell the fishmonger that you wish to cook a whole large piece so that you get a piece off of a new fillet or loin. Then cut it yourself into smaller steaks once you get it home.