I agree with Dill! I went fly fishing for the first time and a lot of my hook sets were when I saw the fish start head shaking - nothing noticed on my line or anything else, just violent underwater headshakes. I tightened up my line to see my little fly in the back of its throat.
Its important to know where your fishing, so seeing the terrain (rocks, logs, etc) not just fish plays a huge part in how you setup your rig.