[HayZee518]

if you have a standard small horseshoe magnet you can find screws, nails, in the studs with it. just attach a short length of string on the magnet and hold it so the magnet swings freely. just move it along the wall and when it spots a screw or nail it'll sort of stick there. then just use a pencil and mark the location. well, you could cut a 45-45-90 triangle from some decent wood and use it as a support. might even add beauty to your shelf [ stained and poly - urethaned]