x2 on test holes
I also prefer to tap the wall with a knuckle and listen for the change in sound...if it sounds hollow, it probably is. Also, most studs are spaced 16" on center (24" if your house was built by a cheap SOB, like mine was). So, if you find one, you should easily be able to find the rest. Also, electrical fixtures (outlets, switches, etc...) are fastened to studs, you just have to figure out which side of the fixture the stud is on.
My crappy little Zircon works well on interior walls only...walls with no insulation in them. Insulation confuses the heck out of it.
***Warning*** the crappy little Zircon cannot tell the difference between a stud and a PVC vent pipe (fortunatly, it was above all of the sinks and plumbing fixtures)