I had heat exhaustion a few years ago on a ride. I knew I was getting in trouble when my heart rate wasn't recovering after any type of effort/climb. Usually it drops quickly - not so on this ride. I found a shady spot in the grass, got comfy, drank everything I had left in the water bottle and then bummed a fresh water bottle from a passing rider. Stayed there about an hour until my HR came back down, then continued on nice and easy.