Call some of the contractor's previous customers and ask if they would hire the contractor again. Ask for proof of insurance. If the contractor does not have disability and workers' compensation insurance, you may be liable for accidents on your property. Ask for a written estimate. Make sure it includes everything you expect the contractor to do.
Some contractors charge a fee for an estimate.
Ask for any guarantees in writing. Guarantees should be written into the contract and should state what is guaranteed, who is responsible and how long the guarantee is valid. Get a copy of the final, signed contract. Once signed, the contract is binding on both you and the contractor. Do not sign off before the job is finished.