Listed below are the signs of Heatstroke as listed by the American Kennel Club:
Heatstroke
Heatstroke can be the serious and often fatal result of a dog's prolonged exposure to excessive heat. Below are the signs of heatstroke and the actions you should take if your dog is overcome.
Early Stages:

• Heavy panting.
• Rapid breathing.
• Excessive drooling.
• Bright red gums and tongue.
• Standing 4-square, posting or spreading out in an attempt to maintain balance.

Advanced Stages:

• White or blue gums.
• Lethargy, unwillingness to move.
• Uncontrollable urination or defecation.
• Labored, noisy breathing.
• Shock.

If your dog begins to exhibit signs of heatstroke, you should immediately try to cool the dog down:

• Apply rubbing alcohol to the dog's paw pads.
• Apply ice packs to the groin area.
• Hose down with water.
• Allow the dog to lick ice chips or drink a small amount of water.
• Offer Pedialyte to restore electrolytes.

Check your dog's temperature regularly during this process. Once the dog's temperature has stabilized at between 100 to 102 degrees, you can stop the cool-down process.
If you cannot get the dog cooled down and you begin to see signs of advanced heatstroke, take the dog to the veterinarian immediately.