Magnets - 1 Handed Stud Finders, No Batteries, Easy, Cheap

Using magnets to locate studs, screws, nails and metal laths. They work really well to find edges, centers and screws also. You don't need to pencil mark things like with a stud detector, you just leave a magnet. There's no erasing of marks when you're done. They also work 1 handed (the stud finder takes 2: 1 for the finder, the other for the pencil).

A magnet stuck to driven screws will also hold your screws, bits and small things while you're working.
They are small and you can keep several in your toolbox for stud finding and other things, like retrieving dropped tools, ...

https://www.youtube.com/watch?v=yynVOIgIMSw