The seafood restaurant only has tuna, salmon, cod, or sea bass
"You know that a seafood restaurant knows what they're doing if they have fresh-shucked oysters. Anything that comes in live and has to be dealt with, processed, and served up in a short time frame means you're dealing with a higher-caliber restaurant. If you see steamed clams, mussels, or shellfish dishes, it tells me the restaurant's doing volume and they're treating their seafood great. If you see sea urchin, or other seafood you might not be totally familiar with that strays from the four major fish (tuna, salmon, cod, or sea bass) -- you know it's a good spot. Shrimp will also be frozen nine times out of 10."
The server can't tell you where the fish is from
"Talk to your server and ask them where the fish is from. The better places will be able to tell you. If the waitstaff doesn't know off-hand, it's not a black mark against them, but they should be able to get you the answer pretty quick, and tell you which state it landed in. If they say, 'It's from the Pacific' -- well, yeah, duh. I'm sure it is. If they can't tell you where it's from, they probably don't get in whole, fresh fish. It's probably portion-cut and frozen."