When buying “fresh” fish, trust your eyes and nose.
Eyes should be bright and clear.
Lift up gill plate. Inside should be bright red. Then stick your nose in. If it smells “fishy”, don’t buy it.
I was in a market in Spain one time when I heard terrible shrieking coming from about two stalls away - it was an abuelita (granny) screaming at the fish man for trying to sell her a fish with sunken eyes! Actually, they weren’t even sunken, just somewhat dry looking. But these ladies won’t take anything less than glossy eyes.