A good shop has a sound room to demonstrate the products. Ask to see demonstration vehicles. Hearing products perform in a sound room is good, but it's even better to hear them in a car. It also lets you inspect the shop's installation techniques. Ask if the shop has a woodworking room.
Wood is used to construct subwoofer enclosures.
If the shop has a woodworking room, it's a good sign that the shop is precise in its work. Ask if the installers have experience working with upholstery. A good shop has installers who are well-skilled in using upholstery to trim their installations to make them look stock. Ask if the shop uses computer software to design and install the sound systems. Talk to the salesperson and try to determine if he or she has experience designing sound systems.