If you were to draw a straight line from the horses pole to the ground, their nose should be at or a little before. If it is overbent they are behind that line
Typically the 3rd vertebrae should be the highest point of the horse's neck if they're on the bit. Anymore than that and they're over bent. But "on the bit" is more than just the head position, it's when the horse is working all through their back and hind quarters which causes the head to come down and work.
bungies are great as well it allows the horse to drop its head out long and raise there back
I use draw reins they seem to help my horse to flex properly