Seeing that we are in salmon season I would say it depends on what you are after. As people have said in other posts, if fishing for Springs it could be as subtle as the line stopping or something is nibbling at the fly. In those cases, set the hook, and hang on. I have had some Chum salmon be tentative biters, but in most cases they slam the fly and take off with it - no doubt when they have bit. For coho, your best success will be in casting to where they are holding and stripping the fly back in. The stripping action is what gets their attention and makes them come out to bite the fly. So as you put the hours on the river you will know when the salmon has bitten your offering. Trout fishing is another can of worms and I won't dwell on that for now. One last thing I will add as it happens at one time or another when you are flyfishing. If you snag a salmon in the tail for instance, point your rod at the fish and give a nice steady pull back to break the fish off. Saves not only on the wear and tear to you and your fly gear, but also to the fish. Hope this helps.