Before electronics made our lives easy, people found the nails or screws in studs with a magnet. I tested this trick out in comparison to an electronic stud finder while installing a set of shelves, and it's doubtful I'll ever use my electronic stud finder again.
Step 1: Grab Magnet From Fridge, Hang With Dental Floss
I've read that people find this method too slow, but the electronic stud finder always leaves me guessing. Once I saw the pattern of fasteners there was no question I had found studs. If you want to speed the process up, a strong and light rare-earth magnet would really seek out the nails.