Freshness is easiest to gauge when the fish is whole. Look for these signs of quality in both whole and “dressed” fish:
Appearance is bright and shiny.
Scales are bright, most of them intact and adhering closely to the skin.
Eyes are bright, clear, and full and often protrude.
Gills are red – and indicatory that oxygen is present and the fish is very fresh. Gill color fades with time to light pink, then gray, and finally to greenish or dull brown.
If the fish has been dressed:
Flesh of most species of lean or white fish should be white, and almost translucent; there should be no signs of reddening along the backbone, nor discoloration of the flesh.
Flesh should feel firm and elastic when pressed with the fingertips. Fish that is limp, soft and remains indented after pressing is stale.
Fish aroma is fresh and mild, not “fishy”.