Spotting frozen seafood in restaurants
As Rumtscho said in her comment, much seafood is frozen on the boat before it even gets ashore, let alone into a restaurant. However, it is possible to keep seafood fresh using another method. Good seafood restaurants will have live food in tanks ready for eating. I often go to such restaurants and pick out my fish or crab from a tank and watch as the waiter nets it out. In my local market I can buy shrimp still jumping about in the bag as I carry them home. If you insist on fresh seafood, then look for places like these. Usually they will make a show of the live fish in tanks at the front of shop/restaurant.

To answer your question: If you don't see it jumping about live before it's cooked then assume it's either frozen then cooked, or cooked then frozen, or maybe cooked then frozen then cooked again.