Is it possible to identify the quality of fish after cooking the fish?
Old filets of fish will be slimy and have an overly fishy smell. When frozen filets are poorly stored, (refrozen/not flash frozen/stored at improper temperature/too old) they have this texture of eating a sponge that makes me wretch, since the delicate emulsion properties of the flesh have been completely broken down. The moisture can be sucked out and the proteinous membranes chewed on like rough, spongey, dough. If you try to skin them, they sometimes come apart in your hands rather than adhere like tissue from a living organism.

When cooked poorly (burnt to a crisp, overseasoned), it's difficult to tell the difference between fresh and old filets.