Horses follow a predictable pattern of symptoms when they overheat. When the horse is under saddle, the signs are harder to see, but a horse that is sweating, acting tired, stumbling, missing jumps, refusing work, having trouble breathing under saddle, or trying to take extra breaths may be overheating.
The first thing you want to do is stop work, take your horse to shade, loosen the girth, and pull the bridle. Pour or sponge cold water or better, ice water, onto the head, neck and chest. This will quickly reverse the signs. If there is wind, the horse will cool faster. If not, a fan helps. Otherwise, lots of cold, even icy water.
For more information, please read the article How to Keep Your Horse Safe in the Heat.