If you get hives after eating fish at a restaurant, how do you know which fish (various fish are cooked using the same pots and pans)?

See an allergist. An allergist can assist you in discovering if you are allergic to one fish, some fish, or all fish. Unfortunately, it's rarely just one fish. You also have to consider a reaction to shellfish either crustaceans or mollusks. The websites of the american academy of allergy & immunology (www. Aaaai. Org) the american college of allergy & immunology (www. Acaai. Org) have find an allergist tools.
Elimination. Can't tell which one, if your assumption about many fishes are cooked using same pot. Often, however, restaurants try to separate their fish dishes. Salmon vs halibut vs seabass etc. Require different cooking methods and ingredients to make their tastes unique. So choosing a specific fish dish which caused your symptoms would most likely be that fish. Be cautious. Check with your doc. Good luck.