What Pack said! Also you can pick up some rocks from the stream and look under them to see the bugs.
How's your entomology? Can you tell the difference between a caddis, mayfly, and a stonefly in both juvenile and adult stages? If you can't, learn them. Click here
to read a bunch of articles written by bug guru, Ralph Cutter.
If you can find a hatch chart for your region, that would be a big help for your fly selection.
In regards to your trout fly selection, keep it simple.
You need big flies.
You need small flies.
You need light colored flies.
You need dark colored flies.
When in doubt, just throw a bugger or a San Juan Worm.