Electronic stud finders are specifically designed to detect density changes or metal behind drywall, accurately locating studs.

Purchase an electronic stud finder (edge-finding or center-finding models available, some also detect metal pipes or live AC wiring).
Place the stud finder flat against the wall.
Turn it on and calibrate it according to its instructions (usually involves holding a button until it beeps/lights up).
Slowly slide the stud finder horizontally across the wall.
The device will indicate (usually with lights and/or sound) when it detects the edge(s) or center of a stud.
Mark the stud location(s) with a pencil.
Verify by finding both edges or by locating the same stud higher or lower on the wall.

N/A - The most accurate and reliable method for locating studs, especially for those less familiar with construction.

Use an Electronic Stud Finder