This, in itself, is some indication of reliability. The dealer should be affiliated with a nationally known manufacturer. As a result, when they make recommendations, you can be more assured they have selected the right system for your home. They are also trained in installation and service. Ask for references.
Former customers are an excellent source of information.
Call these individuals and ask if they were pleased with the dealer's work. Expect an on-site evaluation visit. A good dealer should take a personal, thorough look at your home, evaluate your overall comfort needs and recommend the best system for you. Beware of a dealer that simply takes information over the phone. Check local licenses.