Try to find at least 2 or 3 contractors to interview so you will have a choice. Recommendations from friends or people in the neighborhood are a great place to start.
Established roofing materials distributors are a good source for identifying contractors. They usually know who’s been around and who pays their bills.
It is a good idea to talk to a codes official in your area. They usually know who does good work and who complies with codes without having to be reminded.