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 ...

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 ...

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 ...