I'm with Carl, I go by the gum line. The spots on the tail thing is too hard to tell because unless it is a Sockeye, all of them have some spots on the tail! The gumline will tell you however, if the gumline in white and the mouth is black, Coho. Kings will be black all the way.

Learned this on the Chehalis last year when we were marking Coho as kings because everyone was saying "if it has spots on its tail, it is a king!" Well, when we caught one of each, it was clearly easy to tell by the gumlines. Until we had them side by side, it was hard to tell.