You could take a refrigerator magnet to it and hope that it sticks ... LEDs and with a sound that changes in pitch as the body filler gets deeper. You can even hear the sound in noisy areas ...

Back in the day, it was common to use a magnet to find out where body filler had been applied. Wherever the magnet didn’t stick to the metal panel, that means some sort of non-metallic body ...

A Dremel split one case in half and removed the sides on another, while trusty old car body filler helps fill and ... Using some magnet wire, connections were bridged over to the original ...

This type of filler, of course, took the industry by storm, though at times it gets a bad rap because many a shadetree panel beater or ill-taught body shop tech uses it in a completely wrong manner.

In other words, if there's been rust damage that's been cut or polished off, or body work done to replace entire panels or fill in holes, your magnet won't stick to the spot where that touch-up ...