The first stud finder I ever used was my knuckle, and therefore cost me nothing. An experienced carpenter showed me how to tap on a wall listening for changes in the sound. Once the hollow echo gave way to a more pronounced and solid thud, you could reasonably assume that you were tapping over a stud location. I didn't always have a lot of luck with that technique.
Another carpenter told me that he always drilled holes in a wall, then used a piece of wire to probe inside the cavity for studs. Some pros still swear by this technique, and there is little question that it is effective. But it takes time and leaves you with holes that need to be patched, which is not something a lot of DIYers care to do.
Fortunately, we can buy effective and affordable stud finders that will do the hunting for us. Here are a couple of options.