Use the magnet method. Drywall is nailed on to studs. If you use magents and locate a column of three evenly spaced nails that line up vertically, you can be pretty certain that there's a stud there, especially if there's another column to the left or the right about 16" apart.

See this demonstrated in this instruction on making a DIY magnet stud finder made using a magnet, dental floss, and tape:

Instructables | Inexpensive and reliable stud finder