Here's how to tell if Fish is Fresh:
Bright, Metallic Skin
The main thing you want to look for is skin that has a metallic shine. Any discolored or dull spots indicate that the fish has probably seen better days. If you’re buying a fillet, make sure the flesh has a robust color—it becomes less vibrant as they age.
Meat that Springs Back
Whether you’re dealing with a whole fish or a pre-cut fillet, see if you’re able to test the quality of the meat with your finger. You should be able to press down, but your fingerprint should quickly disappear as the flesh bounces back. If it remains, the fish is no longer fresh.
If you’re buying a whole fish, pay attention to the eyes. They should be clear and slightly bulging. While dull, sunken eyes don’t mean the fish is past its prime, it also won’t be as fresh as possible.
Firmly Connected Scales
You should be able to detect if scales are intact by looking, but if your fishmonger allows it, run your hand along them. They should stay in place and feel firmly connected to the body, almost like armor. If any scales begin to slide off, you’re dealing with an older fish.
The gills can reveal a lot about the freshness of your fish. First, make sure they are bright red and moist, rather than brown or faded. Also, check for any milky liquid around them—if you notice any slime, move on.
No Milky Liquid
When buying fillets, you may notice liquid on the fish. Only purchase if the liquid is clear, since, like with a whole fish’s gills, the milky coloring could be a warning that the fish is beginning to rot.