Say you want to hang a shelf. Knuckling the wallboard can pinpoint a stud. But to better the odds when your electronic stud finder's gone missing, use deductive reasoning. Most studs are placed at 16-inch intervals, so once you know where one is, you can usually find the rest.
Start at a corner, where there's always a stud. Or take the cover plate off an electrical outlet and find out on which side it's mounted to the stud. From there, measure 16, 32, 48 inches, and you should hit a stud at each go. Eliminate all guesswork by using a thin bit to drill a test hole at the top of the base molding, which you can easily repair with a dab of caulk. Simple Stud Finder