Your best bet on this is when you get them stand them up (don't park them out; that covers lots of conformational flaws). Then evaluate them like you would any other horse.
One way is to imitate an old custom allegedly from Arabia. Buyers would sit in a tent that was divided by a curtain, with horse and buyer on opposite sides. The curtain would slowly rise, allowing the buyer to inspect the feet, then the legs, then the bottom line, etc. If an any point the buyer did not like what they saw they said "stop," the curtain was dropped, and the horse replaced with another. If the curtain went all the way up the buy just bought the horse.
So, start with the feet. Are they trimmed to anatomical correctness? Are they shod and if so, how? Are the legs well formed (front and rear)? Etc., etc., etc.
Note that you are judging the horse AS A HORSE not as a Walking Horse. Some minor conformational differences can exist in Walkers (such as a slightly longer rear leg) but that longer leg ought not to have "sickle hocks."
After you've done the conformational analysis make a call. Dismiss the horse, fix what you can fix, or just accept the flaws. But keep them in mind as you work the horse. Now move the horse and see what you get. The probability is that you've got some flavor of pacer. The true running walk is vanishingly rare. Trotters are almost as rare. Remember that movement can be heavily influenced by tack type, fit, and adjustment. It can also be influenced by rider's seat and equitation style. Keep the tack and rider as neutral as possible so that the horse's native way of going can be evaluated.
In all of this watch the temperament. If there is a "silver lining" to the dark cloud of institutional Walker industry cruelty it's that most Walkers have pretty quiet temperaments. But not always. Bucking tendencies are rare; rearing is much more common.
If you get a heavy shod horse with a long toe and low heel in a Walking Horse Bit that's being ridden by a rider with their feet on the dashboard and their butt against the cantle then you have have a major problem. Take great care to make a careful evaluation 'cause addressing problems can get real expensive, real fast. Or, put another way, there's no such thing as a "cheap horse."
You sound like you've got a decent eye. Use it critically and don't get "side tracked" by Industry nonsense about how Walkers are unique with all kinds of "unique conformational characteristics." That's bunk. They are horses first, and Walking Horses second. Evaluate them that way.
Good luck in your project.