Look for the License
The first thing you should check, as you would with any contractor, is that your contractor has the proper licenses and insurance. A license will let you know that your contractor has experience in his or her field, has been checked out with the licensing board, and is bonded. With that knowledge, you can rest easy knowing that you're working with someone who has been in the business for some years and is capable. You can ask the contractor personally for the appropriate credentials, but if you need to feel doubly sure, check with your state's licensing board. After you've checked on the license, you can also make sure that he's insured. Having this insurance means that if anything were to go wrong during the building process, you would have some recourse to recoup the money that you've put into the project.