It would be best to have a veterinarian show you what to look for. Radiographs could be taken, or just bend the knee and feel for open spots in the joint. Again, it's best to have someone who knows how to do this show you the first time or two. Your horse's knee joint as a yearling would not be closed yet, but it would be a good time to begin monitoring its knees so you can feel the difference as the closure occurs.
