I always start checking the gums and if they start getting red, I do something to cool the dog down. Lizzie will pant more than my other dogs, and it took some getting use to. She will also go out, like the other dogs and just plop down in the sun. When they get good and hot or I can't take it any longer, they come in looking like they are having a stroke. Here is a site with the symptoms of heat stroke:
Heat-Related Illnesses