Easy. You look at the bone structure(Which doesn't really change in the big areas, the proportions just change a little. You can accurately judge a horse's conformation at about 6 weeks.), consider what will change, and what doesn't. Most of the most amazing athletes in the equine world(of ALL disciplines) all have three things in common: a "light" shoulder, a neck emergence above the point of shoulder, and the lumbosacral gap placement nearly directly over or right over the point of hip, which allows for the most amount of flexion possible. The light shoulder allows the horse to more easily travel uphill because he doesn't have an excess amount of weight only
resting on the front legs - a horse roughly carries about 60% of their body mass on their front half, but in certain breeds this is less/more due to conformation. The horse with the lighter shoulder can raise himself up better and travel lighter and is all-around more athletic than the horse with the heavy, weighed-down shoulder. The neck emergence above the point of shoulder also makes it easier for the horse to raise the base of his neck and lighten the forehand, in turn - making him even more uphill. Combined, these three traits are what make the superior athlete.
1.) The yellow line drawn through the shoulder is marking the path of the bone structure and where the weight is resting. The more in front of the wither that line emerges from the horse, the lighter on the forehand the horse is due to having less mass weighing him down on the front half that is nearly impossible to shift to the back legs. When the line bisects the wither(as it does typically in a QH), the horse cannot very easily shift that weight back and ligten the forehand as easily, so he is naturally heavier on the forehand. You may note - all of the great dressage horses, barrel horses, and many of the more "English" QH have that line(called the "Pillar of Support") ahead of the wither. Rugged Lark is one of them, Reys Dual Badger is another, Favorite Cartel is another. Negro, the famous WB stallion(sire of Valegro) has a Pillar that is far
ahead of the wither, and it obviously shows up in him and his offspring. This ratio and balance doesn't change due to growth spurts, so once a foal is properly unfolded it can be judged.
2.) The two red lines: the upper line marks the neck emergence, the lower line marks the point of shoulder. The neck emergence is quite obviously above the point of shoulder - a trait carries by all jumpers, dressage horses, most barrel horses(at least, the successful ones), and most of the more versatile QH. (Those who don't have it are always heavy on the forehand, and failing the over reach or even track up at times.) High Brow Cat's neck emergence is above the point of shoulder. ;)
3.) The two light blue lines at the top of the hip: the upper marks the location of the lumbosacral gap(the point of most flexion in the whole spine from withers back), the bottom marks the point of hip. This optimal placement allows the horse to easily get underneath himself and shift weight back onto the hindquarters much easier than a horse who's flexion is impeded by having the LS gap placed too far back. Most of the highest grossing barrel horses and racehorses(as well as all the great dressage horses) have a perfectly placed LS gap.
4.) The blue lines on the shoulder merely mark the slope of the shoulder, which is quite lovely. He'll have a great length of stride and agility.
5.) Lastly - the purple line and the yellow line. If this horse was to be set up square, he'd be standing on his toes on the back feet because he is sickle hocked. The purple line marks where the cannon bone should
fall, the yellow line marks where it is. Quite obviously sickle hocked.