Ways Your Horse Tells You It Needs Its Teeth Checked

cannot chew properly is also prone to choke and impaction colic
If your horse has broken teeth infected gums sore cheeks or any other discomfort in its mouth caused by dental problems it may chew its food very slowly. This can lead to weight loss and poor nutrition especially if it is rushed in any way say by other horses that want to steal its food.
Some horses have no tolerance for pain and will act out by becoming resistant. Some will become spooky and are likely to bolt at the slightest distraction. Others will be quite stoic and will put up with a lot which is why regular dental care is important.
A horse that eats by spilling or throwing grain may be anxious about its food. Also it might be trying to keep an eye out for any other horses that may try to steal its food. Or the horse may just have a difficult time holding the grain in its mouth and chewing because of dental problems. Because the grain might not be ground up or chewed properly choking can result.
If there is a bad odor coming from your horses mouth or nose suspect an infection of the gums or elsewhere in the mouth. This might be the only sign of a potential problem that might require treatment with dental work and antibiotics. 
Horses may drool or slobber after having eaten plants with fungi or some other irritating substance. Also something might be embedded in the gums or under the tongue or the horse might have a dental problem. Some horses drool when they have a bit in and that is pretty normal. But if you are not sure it is probably best to call a vet or equine dentist.