Is there a website that you can check to see if a contractor is licensed and is valid? Is it a state by state thing, or is their a national database as well?
Contractor licenses are normally issued by the state. Get the licence number and call your Secretary of State's Office, usually the corporations and licensing division to check if they are in good standing. Some states require licensed contractors carry liability insurance, some don't, but if you are doing a large project, ask for an insurance binder.