I used a "stud finder". On the windshield of my Chevy Lumina minivan it would not calibrate. On all other windows it worked well.
My 2m rig had very high SWR with the antenna on the windshield. The stud finder helped me locate a spot that would be acceptable which turned out to be the window class on the side by the windshield.
Since then I have used the stud finder several times and it worked well in assisting me in the evaluation of suitability for thru-glass antenna mounts.