Making sure they are licensed and insured is also a must.
I would recommend getting 3 estimates for a new roof. Make sure you compare the estimates apples to apples and that all companies are fully licensed and insured. Also, request that the contractor provides you with an insurance binder that includes you as the certificate holder.
As recommended, you should always get 2-3 estimates to see the price and quality. Always check out the contractor and the work they have done, along with the insurance and license.