Most decent stud finders will show the center for you.
http://www.zircon.com/products/center_ss_i60.html
With stud finders, spending more money on better brands is usually worth it due to better sensing depth.
A magnet will find the nails holding the drywall to the framing, that will be close enough. Or you can go old school and rap on the wall with a knuckle and listen for the change in pitch, that's how I've done it for years.