One old method that I've found fairly accurate is the following:
Measure the distance from the coronet band to the elbow. Double that measurement and you should get the total height of the horse when it's mature. If you measure in inches, simply divide the total by 4 and you'll get the height in hands.
There's another method that involves measuring from the coronet band to the knee, but I can't remember the details... maybe someone else does and will contribute the rest of the information.
I use a method like cattle Annie. Only I like to see exactly how tall they are gonna get. I use a string and like annie said measure from the cornet band to the elbow but then turn that string around and run it up from the elbow to thw withers. This will show you how tall they will be.