It is a real good idea to ask about applicable state and local licenses, insurance for liability and workmen's compensation. Ask for details about the firm's experience, its long-term stability and expertise of its staff. This is especially true if you're adding air conditioning to an existing heating system where ductwork must be added. This case takes more planning and know-how to get the job done. Inquire about warranties.
Warranties vary according to manufacturer.
Make sure the contractor explains the terms of your warranty and that you understand it. Many manufacturers also offer extended warranties, covering parts and labor which you may be wise to investigate. Get a written estimate. To make a fair comparison, be sure the estimates you are looking at are for the services you've requested and are based on the same size and efficiency equipment and the same quality of workmanship. Ask about maintenance contracts.