Ask for references and call the references. Subcontractors take on dozens of projects a years. You should talk to at least of couple of the contractors or owner/builders who have hired them in the past. Make sure that other people have had good experiences with the person before you jump into bed with him.
Ask to meet the crew. Find out how many people are working on the crew and how many other jobs they might be working on at the same time. You want to make sure that they have the manpower available to get your job done on schedule.
After you have a couple of good leads on subcontractors, you should get their bids on your project. Look over the bids carefully. You won't necessarily give the job to the lowest bidder but to the lowest bidder who will be able to give you what you want. Ask for an itemized budget and if the sub doesn't want to itemize the budget, cross them off the list. You should not be paying a blanket fee for "materials" or "supplies"; rather you should know exactly what those materials and supplies are. Check over the budget very carefully to determine the quality of work that you can expect from that particular sub.
After you have selected the bid that you want, there are still a couple of steps to take before awarding the job. You should make sure that the subcontractor has the mandatory certificates and insurances to be working on your project. Subcontractors should be licensed and they should also carry general liability insurance as well as worker's compensation insurance. If the sub tells you they don't have worker's compensation because their employees are "independent contractors" go back to the drawing board. You definitely don't want to be sued if an employee is hurt working on your site. The sub that brings the employee in should be responsible to ensure those employees.