Knowing seasons and local fish helps. If the restaurant is peddling local red snapper, it may not be either one. Grouper has a very limited season in the keys. If they are cooking yellowtail, you can probably be safe it is. Mahi mahi is likely off the truck, as are oysters, tilefish, and probably shrimp. (It's way cheaper to buy it from a place like Ft. Myers where 50 shrimp boats dock than it is in Key West, where 4 shrimp boats dock. There are other problems with local Key West shrimp markets, but that's for another time and place.) If the finer restaurants (915 Duval, Kelly's, Square Grouper, A&B Lobster House, etc.) say it's local, it probably is. If the restaurant has a fish dock behind it, they are likely getting first choice. If you think you're getting local fish at Turtle Krawls or Conch Republic Seafood, well, you're the tourist they are looking for.