Someday contractors may have feedback listed like sellers on ebay where you can just click and see what every previous customer had to say about them. I haven't found any good easy way to sift out the best contractors in my area, but here is what I have tried and my comments.
Brand web sites all have dealer locators, any many will list them out sorted by distance from your area or zip code, and further rank them by type of relationship to the brand. A couple brands also sorted by dealers of special merit, but it wasn't real clear to me what the criteria was, and I found none of the merit dealers in my area. Best I could do was to list the authorized dealers, which I hope got more training or something.
Trade associations also list affiliated dealers, such as nate.com
Better Business Bureau http://www.bbb.com will have a rating on almost everybody, but I think it is best maybe to weed out the worst, not to find the best.
Consumer orgs, I joined http://www.angieslist.com which has consumer ratings for all sorts of service companies, but how effective it is depends on your area and the type of service. I found about 60 contractors rated in my area, all but a few with good or excellent marks, but except for sears none had more than one person giving the rating, and frankly I don't think most consumers are qualified to judge a HVAC install for either quality or price.
Gets some bids and judge the contractors on the quality of the estimate, measurements taken, professional behavior, accuracy of statements they make, and finally compare the bids from the different contractors. This is how I picked my contractor.
I am too lazy to do some of the other stuff I know that maybe I should, get references etc., but I am happy with the bid, they are clean on the BBB, and have been in business 22 years. Works for me, but I do wish something like Angieslist.com had the membership to be more effective (for my area code they have about 27,000 members and this is NOT enough to get good feedback since many don't bother to rate anybody).