Are they are fully licensed, certified and insured for workers compensation and property damage?
If not you might be stuck paying out of pocket for damages and or even vulnerable to potential lawsuits.
Is their bid especially low?
It is human nature to gravitate towards the roofing contractor who offers you the lowest price, but unusually low bids may be indicative of less than desirable materials or shoddy workmanship. Good materials and expert craftsmanship may cost a bit more, but the expense is more than worth it.
Ask the right questions, do your research and keep communication open and you should have no trouble finding a dedicated roofing contractor in your area.