Having been a gunsmith for over 30 years (now retired), I can tell you a few things to look for. First, this is a time of "specialty", meaning most gunsmiths find their nitch and specialize in it. When I started, I made custom rifles, repaired pistols, rifles and shotguns, and the whole gambit. Later, I specialized in S&W revolvers, 1911 pistols, and still did repairs on most other guns. By the time I had 10 years in it, I was turning down work on anything but S&W's and 1911's. I was able to stay very busy and not have to keep a stock of parts for everything that came through the door.
Second, look for an older, more experienced gunsmith. They have made their share of mistakes and can probably do a "job" in much less time, just because they have done that same job many times before. The young man that took over my business was an excellent gunsmith. We worked together for almost a year. I could strip, repair, and reassemble a S&W before he got the side plate off. However, he did a fine job, just not as quick. His charges for labor were always more than mine, because we charged for actual time spent on repairs. Gunsmiths that have been in business for a considerable time are much less likely to screw up your gun or over charge you.
Third, be prepared to wait for good service. Most good gunsmiths are backlogged a few weeks or more. If you walk in and the gunsmith says he can fix your gun immediately, I'd wonder about his quality of workmanship and repeat business.
Ask for estimates. Most gunsmiths will charge for this but it is well worth it. If it's a minor problem, they'll usually credit the estimate cost toward the work. Some extensive repairs or modifications can easily cost more than you paid for the gun.
Heres a good pistolsmith forum with area references
http://www.pistolsmith.com
