Stewmac sells a tool called a fret rocker.  The idea is that you slid this tool along the frets.  If the tool sits flat all the fret are at the same height and life is good.  If the tool rocks a bit, you have found a fret that is higher than the other frets.  This tool works well on the higher frets, but not as well when you get below the five fret.  Has anyone found a tool that has the same rocking motion below the five fret?  (The three fret doesn't count.)