I wonder the same thing about my filly. I told you in another thread what my vet said about their growth at a year old, but I just don't believe it. There's the string test where you measure from the middle of the knee to the coronet. I believe one inch = one hand. (?) Luna measured out to 15 inches. I'm a shorty so I don't really mind if she's on the shorter side, but it would be nice to have an idea.
My Dad always said that if you take a string and measure from the middle of the fetlock to the point of the elbow and double that distance, that is how tall the horse will be. Usually it is accurate within about a half an inch but it is one of those things from a long time ago that is not always right. It should give you some idea though. :)