The best measurements I could find (at least it worked for me). Measure front leg from knee to the coronary band. If it's 15 inch - horse will be 15 hands, 14 inch - 14 hands etc. Better to do after they are 2 years old. I tried it on number of horses for interest (including mine) and it worked very precisely.
Somehow other test described didn't work for my paint. She has very long legs, so the test gave something like 16 hh. She's 15 though and not going to be any much bigger. :)