Horses can't tell us if they're thirsty (well, they can look longingly towards a stream, but...)
How can you tell if your horse is dehydrated? What we generally do is called a pinch test. The horse's skin loses its elasticity. You gently pinch up a skin fold on the back or neck, and see how quickly it relaxes. If it stays pinched, the horse is dehydrated and needs water as soon as possible.
(Even better, is a 50/50 mix of water and gatorade. Don't give a horse undiluted gatorade - they won't drink it - but if you dilute it they will and it's much cheaper than horse electrolytes).