One sure way to tell is there is a little creature called a copapod that attach themselves in the mouth of a steelhead. The steelhead has to be in the river for a little while before they get them. This time of year the summers will have them and the fresh winters won't. If you look carefuly inside the mouth they look like tiny brown bugs about 1/4 long. A fish biologist told me about this and it seems to make sense after catching several and checking it out.