I try to ride early in the morning before it gets too hot. I offer water before and after the ride (during the ride if out for a long time), monitor respiration rate, pinch the skin on her neck to check for dehydration, and I pay attention to my horse's behavior and level of responsiveness. After riding, I'll often hand walk her to cool her out, rinse her with cool water, and put her somewhere with a breeze while she's drying off.
I'm curious to see how everyone else deals with extreme summertime temperatures. What warning signs do you look for to see if your horse is overheating? At what temperature do you make the decision that it's too hot to ride? Any input or related info is appreciated!