Well, just to confuse the issue, if you colour all the fret tops, and there is only one high fret, when you put a straight-edge/file on the neck, you will find it marks 2 frets - the high one, and whichever one is nearest one end (or the other) of the edge/file (depending on which way you "rock" the straight edge and how long the straight-edge/file is). Also, with 2 high frets, if they are close together, you may still only find one of them, because the angle of the edge/file makes it climb over, and miss the second high fret that is hiding just next to the other high one..
I read one description of fret levelling, that puts it differently. They said to mark the frets, then file them down until the marks disappear off all of them. The lowest frets will retain the marks longest. When all marks are gone, they are level.