Make sure you see the contractors Workers' Compensation certificate. Before making the final payment make sure the Workers' Compensation has been paid. Never pay in cash. A commitment to safety indicates an overall commitment to quality. If it sounds too good to be true, it probably is.
Details are important.
Have the contractor explain his project supervision and quality control procedures. Request the name of the person who will be in charge on your project. Ask for copies of all material and workmanship guarantees and warranties. Cleaning of gutters and yard clean-up should also be part of the deal. If they don't write it down, it may not happen.