ask for recommendations
then visit the one you choose
look round the premises at what they have in at the moment
look at the quality of some of their recent work
make your decision
i prefer places that have manufacturer approval from the big companies for repair work....ie porsche, ferarri, mercedes etc and places that do a bit of resto work too. The place I used had an e type having a full body resto and the attention it was getting was unbelieveable. I was sold when I saw that
Also, i've never seen a good paintshop that wasnt clean and tidy