The most important criteria when you’re buying fish are smell, sight and touch.

If it smells fishy, it will taste fishy, and it is low quality. Quality fish smells like watermelon or is odorless. In addition, the fillets should look fresh – no dried edges or obvious oxidation. Finally, the fish should have a firm, solid texture. After touching it, you should not be left with a slick film on your hands. When in doubt, buy a small quantity for "experimenting."

The same criteria apply to frozen fish when thawed.