Since magnets are not magnets, this is hard to do these days. Since the rare earth mineral market has been taken over by the Chinese, yesterdays magnets are not the same as todays magnets, which over time, yesterdays magnets have gotten weaker when todays magnets have gotten erroneous. In other words, its not a good indication *but better than nothing* of bondo depth.
Its common practice on body parts to skim about an 1/8th inch of filler on the part, then sand to smooth perfection. I think the moral of the story is not to gauge the quality of your truck by the amount of bondo, but the smoothness of the work.
The reason as I have found many times over is that fenders, hoods, cabs, etc from the 1950's have lots of very very Unnoticeable hail, rock, USE damage over them that you simply cant see but when you skim coat the entire surface, then sand properly, you wont even believe how necessary the skim coat is.
Bondo is your friend if used properly.
