You can check your horses for dehydration if you are worried that they aren't drinking.
How to check for dehydration:
1) Skin pinch test: Gently pinch your horses skin in the middle of their neck. If the horse is dehydrated, the skin will not flatten and relax right away and can take upto 10 seconds to return to normal. If your horse is adequately hydrated, the skin will fall back into place right away.
2) Capillary refill time: Gently press your finger or thumb into your horses gums. If the horse is dehydrated, the gums will stay a white/pale for a little bit before returning to the healthy pink colour it is supposed to be.
You could also offer the horses in the field free choice salt (loose salt, not salt block), which will encourage water intake. It is an inexpensive way to help promote healthy hydration in the herd throughout the winter months.