Good air circulation is essential in regulating dog's body temperature, which prevent excessive pants. (practical-pet-care.com)
Signs of heat distress include excessive panting with the tongue fully extended, stumbling, glazed eyes, disorientation, hiding in the foot well (which is the «coolest» place in a vehicle), convulsions / seizuring, and ultimately coma and death. (ottawahumane.ca)
Breathlessness is the most common sign, starting as excessive panting with exercise. (cavaliercampaign.com)