There are a number of keys to deciding on the right fly. One of these is the rise form. Here is a very quick and dirty breakdown:

1. The Caddis rise: A very hard/loud rise - Sometimes jumping out of the water. The harder and louder the rise the more likely the fish is chasing a caddis. Caddis seem to be able to get out of the water faster than any mayfly and the trout know they don't have much time to catch dinner.
2. The Bubble/Dun Rise: The trout leaves behind a small bubble on the surface of the water. In this case the trout is usually taking some mayfly dun. This rise is slower than the Caddis rise, but not as slow as the sipper/spinner rise.
3. The Emerger/Porpoise Rise: You see the back and the tail, but never the snout of the trout break the surface of the water. The trout is taking the emergers just below the surface of the water.
4. The slow leisurely delicate sipping/spinner rise. These rises are the slowest. The fish almost seems to be out for a leisurely stroll. I once put a trico spinner on to throw at what I thought was a 9"-10" trout. That fish turned out to be a 19 inch brown. Very often these rises occur more sporadically, especially as the density of spinners falls on the water falls.

Now since I like to fish dries I'm quite often throwing spinners if there is no recognizable hatch happening. So if your bored of nymphing try casting a rusty spinner. You maybe pleasantly suprised at the fish you catch.