Some say this works:
Measure from the mid-knee to coronet line.
It can be off an inch or so, but usually not more.
14 inches = 14 hands....14 1/2 = 14.2.....15 = 15 hands... 151/2 = 15.2...wetc.
The coronet is the band around the top of the hoof from which the hoof wall grows.
When this method was used on the adult horses as a test it was found that by laying the string while following the contours of the leg, that the measurement matched their real height.
It's important to understand that even if this measurement is accurate, it depends on the health of the horse and especially on the quality of feed it receives during maturation period...