Number one is the quality of the collision repair that the shop does, most shops nowadays have a website that usually has testimonials from customers and/or before and after pictures of vehicles they repaired.
Also be sure to look for certification at the shop (I-car, ASE certification or any frame machine certification). The longer a technician has been doing body repairs also contributes to the knowledge and experience they have with auto body repairs.
I also look for a clean and presentable facility (if the office looks like a mess, then usually, the shop itself will be in a worse condition) office staff should be friendly and greet you with a smile.