Contacting others who have worked with the contractor in the past is a good way to determine his reliability and to get an idea of what the roofing experience would be like with this contractor taking care of the job. However, realize that the roofer can legitimately refuse to give a long list--many customers may not want their names released.
Needless to say, longer is usually better than shorter. Less than three years may signal an unstable business. On the other hand, everybody has to start sometime. References will be helpful to double-check any business, and are especially important when dealing with a new business. A newer business may have a great future, but it is only reasonable to be more careful when considering its referrals.
If your contractor does hire out a subcontractor, it is a good idea to go over all of the same questions with them. Of particular interest is insurance; be sure that the subcontractor holds all of the proper insurance so that you are not held liable for any accident that may occur on the job.
Sadly, most roofing contractors will not offer a maintenance program for your roof once it is installed. Regular inspection of the new roof will allow for potential problems to be caught early and remedied before they cause substantial damage.
Some major litigation could put a company out of business. If a lawsuit is pending, find out what the suit entails. This may include going to the local courthouse and looking at the court documents filed for the case to date.