Practicing this on a putting green with a known break will help you determine whether you're performing the plumb bob technique correctly.
Trust your eyes when it comes to the plumb bob technique. To the naked eye, you may have a hard time seeing the break, but if you see it when you plumb bob, you know it's there.
Do not use this method on a putt that has multiple breaks. Plumb bobbing a putt with more than one break will show only which way the ball will travel on the break closest to the hole.