A picture of a horse measuring stick...

I couldn't find any better picture, not that I looked that hard, but at least you can get the idea.

http://www.wildhorsetrainer.co.uk/for-the-yard-or-stable/stable-equipment/horse-measuring-stick-walking-stick/prod_608.html

The trick I always had with these sticks is if the horse doesn't appreciate your efforts with the stick, they tend to go sideways to avoid them. Just don't be in the way! However, it is the most accurate way.