You've asked one of the most common questions in flyfishing. Sometimes just sitting back and observing will let you know what the right fly for the moment is.
Are there any insects hatching? If so, look for rises in the water. Bulging rises indicate that trout are taking emergers, and true rises to duns usually leave a bit of an air bubble on the surface. If no insects appear to be hatching, turn over some stones or rocks in the streambed and look for nymphs. Note the size and color, and fish a nymph pattern to match. Depending on the time of year, terrestrial insects can make up an important part of the trout's diet.....again, taking note of your surroundings can pay off.
If all else fails, I usually start with a searching pattern such as a streamer or nymph. Even if a fish doesn't actually take the fly, it may show itself and betray its position in the water. This is very common, especially with streamers. That same fish can now be tried for with other flies.
I'll let others who are more familiar with your home waters chime in with particulars, but this should get you thinking in the right direction.