Watch from behind the horse as it's trotted to you and away from you. Ask the seller to do that. You can then see how the feet land, evenly? Or, is one foot landing farther to the in or outside of a level and square track; do the hocks swivel, does the horse wing or paddle in front? Does he carry his tail relaxed and even? Does the horse show a hike in the leg or hip, does one hock or hip travel higher or lower than the other? Then, from the side, do the hind hooves land heel first, or does the hoof land toe first, indicating a stifle issue? Does the horse travel relaxed or does his head rise or fall when a certain leg comes off the ground, or hits the ground? Do all his joints flex equally, or does one joint have restricted motion so that when the hoof lands, there is a definite heavy sound to the footfall? Do the hind hooves track up and almost land in the hoofprints of front hoof? Walk around the horse and look for any uneven muscling. Loaded shoulders:shoulders that are very tightly muscled can indicate hind or front end, even back issues and they get that way from the horse using his shoulder strength to compensate for weakness elsewhere. That's all I can think of right now.