Look for bright, clear eyes. The eyes are the window to a truly fresh fish, for they fade quickly into gray dullness. Dull-eyed fish may be safe to eat, but they are past their prime.
Next look at the fish. Does it shine? Does it look metallic and clean? Or has it dulled or has discolored patches on it? If so, it is marginal.
Smell it. A fresh fish should smell like clean water, or a touch briny or even like cucumbers. Under no circumstances should you buy a nasty smelling fish. Cooking won't improve it.
Look at the gills. They should be a rich red. If the fish is old, they will turn the color of faded brick.
Look for vibrant flesh. All fish fade as they age. If the fillet still has skin, that skin should look as pristine as the skin on an equally good whole fish – shiny and metallic.
Smell it. The smell test is especially important with fillets. They should have no pungent aromas.
Is there liquid on the meat? If so, that liquid should be clear, not milky. Milky liquid on a fillet is the first stage of rot.
If the fishmonger lets you, press the meat with your finger. It should be resilient enough so your indentation disappears. If your fingerprint remains, move on.
The best way to choose a live fish or crab or lobster is to look for, well, life. Is it scampering around in its tank? Swimming happily? Or is it sulking in a corner or hanging motionless and panting? If so, don’t buy it. Lobsters and crabs starve themselves in tanks and often can be almost empty inside when you crack open one that’s been imprisoned in a tank for weeks.
Your best bet is to make friends with the fishmonger and find out when the new shipments arrive. Plan on being there to meet it and buy then. You will be rewarded for your extra effort.
Buy only at the finest fish markets. These are the places where turnover is so rapid you can be assured of fresh mussels, clams or oysters. You may still get a dead one, but the ratio will be far lower.
What is a dead one? Shellfish are sold alive, so they should react to you. Put them on the countertop and back away for a moment. Then tap the shell: It should close tighter than it was. Oysters are a little tough to do this with, but clams and mussels will definitely react. You can also tell a dead shellfish after you’ve cooked them all. Dead ones do not open after being cooked. Throw them away.
Scallops, a Special Case
Scallops are almost always sold shucked, so what you are looking for are “dry packed” scallops, meaning they are not shipped and stored in brine. Those scallops you see wallowing in milky ick? Leave them be. Better to buy frozen, vacuum-sealed scallops, which are perfectly good by the way, than an inferior wet-packed scallop.
This one is easy. Buy them whole and frozen. Whole because the shell protects them from the rigors of being frozen without losing too much moisture, and frozen because shrimp cook – and rot – very rapidly.
Should you be near a shrimping region, or have access to truly magnificent fresh shrimp, by all means buy them. Head on if possible. Why? Because head-on shrimp stay moister. Remember: Nothing says boring like a dry, overcooked shrimp.
Everything I said about shrimp applies to crayfish, too. Unless you can get them live, in which case follow the instructions for lobsters or crabs.
Squid or Octopus
These are almost always sold to the wholesaler pre-frozen, so you should buy them frozen. Both squid, commonly known as calamari, and its more richly flavored cousin the octopus freeze exceptionally well.
Again, if you can buy squid and octopus – not to mention cuttlefish – fresh, do it! They are rare treats even at fine fish markets and should be appreciated as such. Like finfish, you should look first at their eyes, which should be clean and bright.
The Fish Market
Finally, a comment on the shop itself. The absolute bottom line is that a fish shop should not stink. I’ll say it again: If you walk into a fish market and it reeks, turn around and leave. Fantastic fish can be had at a farmer’s market stall, a hole in the wall or in a flashy boutique, but none should smell like low tide. Ever.