Watch out for:
Dry or pale gums.
Deep and rapid breathing.
Prevent these early signs of overheating from even happening by:
Being careful not to overexert your dog when enjoying the day.
Making sure your dog has a shady spot where he can rest.
Having a lot of drinking water handy if your dog shows any of the above signs of overheating or overexertion.
Putting cloths soaked in cold water around his paws, in his armpits and on his head and neck, as one veterinarian suggests.
Also, don't assume that just because your dog is near a body of water that he can't get overheated. And always watch him if he's swimming - not all dogs are great at doggy paddling.