In order to buy a permit, the contractor needs a Notice of Commencement signed and notarized by the owner; so if the work is started without one, start to worry. Most contractors will run the legal for you and provide a notary to come to your home and notarize your signature. They then file the Notice of Commencement and purchase the permit. The permit should be posted on your home for you to see. If you are unsure, call the building department and make sure a permit was purchased before you pay your bill. If you are looking to go about this on your own and you are the owner of the home, then all you have to do is head to your building department and request a permit for a reroof project.
You will need roofing contractors information, unless this is DIY project. You will need a permit pulled, and inspections to be scheduled to verify and approve the completed work. Doing the job right the first time will keep you protected during this process.