Is the horse at the forefront?
Whether the horse is on the forehand or carrys the weight on the hind can not be determined from the neck's position. A horse that goes in competition frame, with the neck high and slightly in front of the vertical, can easily still go with the weight on the forehand. So even if everything in front of the saddle looks perfect, that is not necessarily the case. A horse that goes with the head and neck in a deep stretch, even very deep position, may well carry weight on the hind end.
What you can always be sure of is that a horse that goes behind the vertical will always go with the weight of the forhand and it applies even though it may have just been competing at the Olympics. It is quite easy to look at pictures and if you are in doubt you can try to keep your hand over the neck and head. Now I will not show pictures of other equipager here on the page. But try to go on Google and search for example. Dressage World Cup and look at the pictures. If one gets into doubt and thinks, this horse look uphill, then take a extra look on the back. It will often hang down and wither and hip will be up. When this happens the horse is still not working correctly from behind. The wither must be higher than the hip.
A young horse or a badly traind horse will always be on the forehand when you let it stretch, it will also be on the forehand when it does not stretch. It is through the training that it will learn to carry more weight on the hind. During a period it will be straight with the weight equally spread on front and hind end and sometimes fall back on the forehand, but in time it will be better and better with the right training. When the horse really begins to step deep under itself, it will, after a while look as if it goes uphill, even if it stretches deep.
Although the horse for example has begun to walk with the weight on the hind end you shouldn't expect it to hold it when you begin trotting. It will most likely fall on the forhand again and the training therefore starts over again but now in trot. And thats how it continues.
For a long time it will be a bit of a mixture where you can momentarily get the horse to work with weight on the hind end, but if the muscles are not yet strong enough then it will fall back on the forehand again. But in time you will experience that the horse can hold it for longer and longer.
Many are of that opinion that the horse should be away from going on the forehand as soon as possible because it is wearing it. Unfortunately, you cannot move the gravitational point by making the horse go higher with the neck. If you do so it will still be on the forehand. Instead, make sure the horse is trained well from the ground before you start riding it. When you start riding you need to concentrate mostly on the walk, as it is in this gait that there is least pressure on the joints. One tries out with the trot and later canter, but if you feel that the horse goes on the forehand, then keep these intervals short.
I hope the pictures help to give an understanding that the horse's head and neck position have nothing to do with whether the horse goes on the forehand. You simply have to look at the back.