How to hire a good drywall contractor

Make sure the Drywaller has workman's compensation and liability insurance. It is well within the contractors or homeowners right to ask for this proof of documentation. Liability protects the job site from any damage that might occur, and workman's compensation protects from any potential lawsuits that might happen due to injury on the job site.
Ask the drywaller for refences. Any good drywaller will have no problem supplying references of past jobs.
Make sure you pay the drywaller after the job is complete. Progress payments are acceptable but keep in mind that once someone has the money in their hands it's more difficult to get them to comeback to finish the job. It's best to pay for the job at the time of completion of the job. This way both you and the contractor know that the job is complete and are happy with the quality finish of the job site. We regularly receive phone calls from homeowners or contractors who tell us that after they made payment to another drywaller that the drywaller never returned to finish the job.
Don't be afraid to research multiple companies and ask questions regarding the process. The best way to know a drywaller before he starts your job is to speak with him, meet with him, and ask plenty of questions before deciding whether to go with him.