Originally Posted by Supermane
Actually, just because a horse withers are higher than his butt doesn't mean that the horses build is uphill. A lot of TB who are built downhill will have huge withers, which will make up for the difference, but they still wouldn't be considered uphill horses. Conformation ? Uphill/Downhill... - Chronicle Forums
^^Here is an interesting discussion on it
What you want to look at is the line between the stifle of the horse and the elbow. A downhill horse will have his stifle higher than his elbows (sometimes a bit, sometimes dramatically) an uphill horse will have their elbows higher than their stifles.
Downhill horses tend to have a tendency to fall flat on the forehand and have a bit more trouble sitting back on their hind end, which is why dressage people look for mounts that are at the very least level if not uphill
But what should be most important is how the horse moves. Some downhill horses can still move in an uphill way :) so just gbecause your horse is downhill doesn't necessarily mean he won't move well!