The ability to lose heat is related to body size and shape. Physiologically, heat retention is a function of body mass or volume and heat loss is function of body surface area. A big heavy horse will overheat faster than a small fit one. It is also why most Olympic three-day event horses are Thoroughbreds or Thoroughbred crosses as opposed to strictly warmbloods. Signs of heatstroke are unexpected fatigue, possibly stumbling, and an increased respiratory rate.
Horses normally lose heat by sweating.
When they are overheated, they pant in order to lose more heat. If the respiratory rate becomes higher than the heart rate, especially after resting for a few minutes, then the horse is overheated and needs help. When this occurs, cool the horse off immediately. Get it in shade. Offer some drinking water.