Well, my first horse was left sided and it took me awhile to figure out why I had to do everything backwards from what the books said. I never knew about the whorls though. I think my current horse is right sided but I'll check to see if this method works. It's pretty easy to tell though so I don't know why this method would be needed.
This is really interesting and I found a whole series of videos by Chris Irwin on the subject at http://www.horsevideolibrary.com!
I have six horses-three quarters and three paints. The whorls comments held up with my horses. That is,if the wholes are between the eyes, the calmer the horse--the higher the wholes the more high strung. Also, there are whorls on each side of the horses neck--the closer to the ears, the quieter the horse and the one closest to the ear is the softest side