Check the eyes
Before you handle the fish, check the eyes. They should be clear, plump and shiny. If the eyes look good, you can determine with reasonable confidence that the fish is fresh.
Check the fins
The tail and dorsal fins of the fish should be wet, and intact. A fish that’s been mishandled will have torn or ragged fins. Older fish will have dry and brittle fins.
Check the gills
Check the gills for color, when first caught, a fish’s gills usually appear bright red, and slowly darken over time. The brighter the color, the fresher the fish. The gills should also feel clean and cold, not slimy.
Check the fila for cracks and pooling water
Look for cracks in the filet that run between the muscles and connective tissue of the fish. Breaks in the muscle itself tend to indicate mishandling. Pooling water inside the container usually indicates that the fish is aging and losing its ability to hold moisture.
Find out how it was frozen
Frozen fish often gets a bad name because companies wait until a fish has already aged to freeze it. Frozen or not bad fish is still bad fish. If a fish is frozen right after it’s caught, it’ll still be good when thawed. It should be frozen very quickly, preferably as soon after it is caught.