Pay Special Attention to the Fish's Appearance
Take a close look at how the fish appears. When looking at a whole fish, the clarity of the fish's eyes can be an indicator of freshness for many types of fish. For these fish, the eyes should be clear rather than cloudy. Some fish, such as grouper and walleye pike, have naturally cloudy eyes. You may need to do some light research to determine the correct eye clarity of your fish of choice.
Fresh fill will also have red gills without any mucous as well as shiny, bright skin. While you may think that bloody fish gills indicates poor handling, bright red blood actually is an indicator of freshness.
Skinned fresh fish should not be dry or discolored around the edges. Each piece should look moist and firm without looking mushy. Regardless of whether the fish is whole or in pieces, the fish shouldn't look slimy or have any dark spots or unusual coloring.
Buying Live Crabs and Lobsters
You are in luck if you have the opportunity to buy live shellfish such as crabs, lobsters, oysters, muscles or clams. If you are interested in purchasing live crabs or lobsters, make sure that the crabs or lobsters that you buy are still moving around or at least can move their legs. These creatures can spoil quickly after they die, so it's important to cook them quickly after your purchase.
Buying Fresh Fish & Live Clams, Mussels and Oysters
You can also use tricks to figure out if clams, mussels and oysters are alive even though these shellfish can't kick. These shellfish will close up shop when tapped. In other words, oysters, clams or mussels with open shells are still living if their shells close when you tap the outside of the shells. Don't forget to tap ones that are only slightly open. This is the normal "openness" for these kinds of shellfish when the shellfish are "relaxed."
Clams and mussels don't have to be completely submerged in water to stay alive. They will live if they are kept damp. Regardless of whether you are looking at mussels, clams or oysters, avoid ones that have cracked or broken shells. These animals are probably already dead.