As a test, the rider can soften his contact, and the horse will maintain the pressure and follow the bit downward. The horse does not have to have his head perfectly perpendicular to the ground; it is acceptable in dressage tests to have the nose slightly in front of the vertical. Behind the vertical is not acceptable in dressage tests, which is a pity because it reinforces the belief that "being on the bit" would mean the same thing as "on the vertical".
Many wrongly believe a horse to be "on the bit" if his head is held "at the vertical," or perpendicular to the ground. However, a horse is still able to maintain this headset while remaining stiff, heavy on the bit, and unresponsive to the rider's aids. The vertical headset is not a guarantee by any means that the animal is truly on the bit, and many novice riders achieve the vertical headset, while losing the impulsion from the horse, because they ride "front to back," or pull the horse's head down in an effort to make the horse appear to be accepting the aids. This is also sometimes seen when the horse is ridden in certain gadgets, such as draw reins, especially if the rider is not skilled enough to correctly use the piece of equipment.
A horse that is on his forehand or unbalanced will not be able to come correctly on the bit, and will usually either lean on the rider's hands, placing too much pressure on the bit, pull against the rider and "root," or brace upward against rein pressure and come "above of the bit." This makes the contact heavy, and the aids can not come "through."
Some horses will avoid contact with the bit, rather than correctly accepting it, and come "behind the bit." This may occur due to evasion by the horse—so he does not have to listen to the rider—or because the rider is using the bit too strongly or physically trying to pull the horse on the bit. It is a very common fault if the rider "see-saws" on the reins. Sometimes the horse will have a very strong contact, most commonly if his head is purposefully pulled in by the rider. Additionally, the horse will bring his nose closer to his chest, or "behind the vertical."
The most important test is if the horse will follow the contact forward and down if the reins are softened by the rider. If the horse follows, it is so to speak the horse that chooses to touch the rider with its mouth. If this quality of contact is established, the horse is really working on the bit, even if its head is a little in front of or behind the vertical.