How do you tell if a car has a bunch of bondo on it? magnet?

This is a toughie. For a couple of reasons, first, if it has filler on it, it doesn't mean that it is a bad thing. Second, how thick it is is kinda hard to tell.

You can get a cheap mil guage that will tell you up to 12 mils or so. They are only about twenty bucks and work pretty well.

You really should bring the car to a trusted autobody shop and let them look it over. There are MUCH worse things than a little plastic filler. An experianced bodyman can spot things that no magnet is going to see.

Brian