By law, contractors must show their license number in anything that might be considered advertising.
Vehicle magnets can be easily removed, so this may be a telltale sign of a fly by night contractor.
Or do they work out of their living room? Having a physical office location is always a good sign for any reliable contractor.
Does the contractor specialize in a wide variety of work? When you are having work done on your roof, you want to work with someone who specializes in roofing, not someone who works with roofing, kitchens, and landscaping.
Beware of extremely low prices. Competitive pricing is one thing, but an offer that is much lower than any other quotes you have received can be a warning sign. Overhead is expensive when done properly, but it helps to protect you as a homeowner.