Ask the installer if he'll remove your panels (even if you aren't going to get him to), if not, walk.
Ask to see his work bay, if it's messy/dirty, walk.
If you don't like the guy, walk.
The installer recommended by everyone here screwed up my car, scratched the tint, stuff in between the tint, etc. I didn't feel comfortable with the guy, but went with everyone's recommendation... Now we're going to court.