Before the advent of these fancy capacitance stud finders, you could buy a mechanical stud finder. They didn't work so well at parties, but if you dug through a man's garage and found one, chances are he's a stud. Seriously, though.
The old stud finders were a magnet at the end of a little red balsa needle, suspended on a loose hinge in a small wooden or plastic housing. You slide the stud finder across the wall, and when it finds a stud, the needle, which had previously been hanging flaccid, springs to life and points straight out. A little homoerotic to be sure. Anyhow, it works because all nails since the dawn of time have been ferrous, and the magnet senses the nails or screws in the stud.
These archaic pieces of equipment will stop working when nails are no longer made of steel and iron; since steel workers and ironmongers are protected by a very strong lobby, these stud finders will work until the Ferrous Bureau's Day Off.
Danke schoen... darlin, danke schoen...