Unfortunately the real proof is how they behave when you put in a claim. You can have an insurance company for 20 years and be "happy" but if you have never dealt with a claim, had them send you to a bad body shop, pay off the bare minimum of what your car may be worth, you'll never really know how good they are.

A good way to find out if you really want to go through the effort is to visit several good local body shops, talk to the manager, and see what insurance companies they do or don't like to deal with. That's where you may find out why the cheap ones are cheap.