Besides visual examination, is there some way to find out reliably how much a given car is affected by rust? I mean, which parts (possibly covered by paint) are rusting in the inside, and to what extension?
I'm aware that there are instruments for measuring paint thickness, but as far as I understand they do not tell if the part below the paint is rusty or not. I've tried to do some research, and it really seems to me that the best one can do is to visually examine the car (also from below, by lifting it), and from the rust spots, try to extrapolate how much the car might be rusty on the inside.
Is this really the best one can do, or are there any other cheap and non-destructive ways which give a more precise result than visual examination? (I.e. some kind of instrumented measuring.)