Most trades are licensed through a state governmental agency such as a "home improvement commission". You'll need to find the agency in your state and verify that the tradesmen is, in fact, licensed and in good standing.
Is the majority of your work in repairs or installation?
Tradesmen such as heating contractors price services in three distinct ways: 1. Time and Material - an hourly rate plus the cost of the materials (which may be marked up). 2. Flat rate - repair and replace a condenser $350 3. Bid - tradesman provides a comprehensive quote upfront for what the charges will be which includes all labor and material charges.