I am considering purchasing a 70 chevelle that I am told has the original quarters. The owner is not sure how much, if any, bondo or filler is in them. I have heard of using a magnet but have never tried this myself. Does this work? how much filler before the magnet won't stick. etc. any help in this area would be greatly appreciated.

I use a flat buisness card size refridgerator magnet. You can get a feel on how much filler is involved by how much the magnet sticks.

You can use the seat of the pants "feel how hard it sticks method". Or you can also search around for a little tool that has a spring loaded magnet inside and a gauge readout depending on how hard the magnet pulls to the metal.

If you are really concerned, look for a "paint thickness gauge" it's the same principle but more finely tuned so you get a readout in .001 inches.

Both of these assume, though, that the seller is going to let you go around slapping magnets or tools all over the paint of their car.