At The Shop

Look for a neat, well-organized facility, with vehicles in the parking lot equal in value to your own and modern equipment in the service bays. 
Professionally run establishments will have a courteous, helpful staff. The service writer should be willing to answer all of your questions. 
Feel free to ask for the names of a few customers. Call them. 
All policies (labor rates, guarantees, methods of payment, etc.) should be posted and/or explained to your satisfaction. 
Ask if the shop customarily handles your vehicle make and model. Some facilities specialize only in certain makes. 
Ask if the shop usually does your type of repair, especially if you need major work.

Look for signs of professionalism in the customer service area: civic and community service awards, membership in the Better Business Bureau, AAA-Approved Auto Repair status, customer service awards.