When buying a whole fish look for clear eyes, tight skin, and an equally clean smell. No fishy smell!
If buying a filet look for a bright, clear appearance. The filet should look "moist" having a genuine glisten. There should be no gaps in the surface of the fish, it should be smooth and even.
Look for fish that has been caught with "long-line" as opposed to "net-caught." Long-line fish are handled individually, therefore better, than those caught in nets.
Find a fishmonger that will respect your business. Ask questions! (Where did the fish come from? How was it caught? What method of fishing was used?) If you like the quality of the fish let them know, they want your repeat business!