To predict a foal's mature height, at the age of six months, measure from the coronary band to the middle of the knee. If the tape reads 15 inches, the horse should be 15 hands high. If the tape reads 13 1/2 inches, the horse should be 13 1/2 hands high.
Alternately, you can tape measure the distance from the point at the rear of the front fetlock to the point of the elbow and then invert the tape to see how many more inches above the withers the horse will grow.
This may not work with ponies, however, since they have a shorter cannon bone.