The best method of assessing the freshness or spoilage of fish is by sensory evaluation techniques. It is recommended that appropriate sensory evaluation criteria be used to evaluate the acceptability of fish and to eliminate fish showing loss of essential quality provisions of the appropriate Codex Standards. As an example, fresh whitefish species are considered unacceptable when showing the following characteristics:

Skin/slime — dull, gritty colors with yellow–brown dotting slime.
Eyes — concave, opaque, sunken, discolored.
Gills — gray–brown or bleached, slime opaque yellow, thick or clotting.
Odor — flesh odor amines, ammonia, milky lactic, sulphide, fecal, putrid,
rancid.

source:

Code of Practice for fish and fishery products
First edition
© FAO and WHO 2009