Magnets work to a point. They will not stick if there is no metal behind, but they might not stick to a very nice metal patch repair with some filler above it for final shape either. Magnets also vary in their strength so if your magnet is strong it will stick over a lot more filler than a weaker one. A mil gauge tool is better but not as available. I use a magnet and a bright flashlight as well as an icepick, (for poking at suspected weak spots under the car if the seller allows me to) and lots of looking closely at many many angles as well as tapping on the normal rust areas when I inspect cars (you can often hear repairs if you know how to listen).
Currently working full time repairing Corvairs and restoring old cars.
Located in Snellville, Georgia
For example, a can of plastic filler (Bondo is a brand name) has directions and warnings on how their product is to be used. When it's used as directed proper paint thickness gauges will still read a thickness. Once you ignore the directions/warning you are now opening up a can called 'Liability' because you are 'knowingly' doing something wrong. And (depending on the state) the same laws apply if you did that when remodeling your home or anything else. And the directions are on pretty much everything you use when 'restoring' your car. Filler, paint, primer, sand paper, etc...
And if you bought a car like that and discover such repairs, you're obligated to let the next person know or you're just as liable as the person that did the repairs to begin with.
And the lawyers taught my dad that backyarders have 401k's, college funds, equity in their home, savings, etc... But, nothing we ever looked at made it to the courts because everyone was quick to settle out of court. No one wanted jeopardize their life savings by arguing in court. Imagine that.
Hiring someone to look at the car, magnets, paint thickness gauges, flashlights, rubber mallots, and knocking around are the best ways when looking over the body and structure.
Not a pleasant subject at times. And if you were wondering when reading this if my dad wasn't well liked, you'd be correct. Certain shop owners didn't care for him and the insurance companies hated him. But, we saved our customers money and got some junk off the streets and into the salvage yards... where the next guy probably bought it and 'restored it'...