Some signs of heatstroke in a dog are heavy panting, glazed eyes, a rapid heartbeat, difficulty breathing, excessive thirst, lethargy, fever, dizziness, lack of coordination, profuse salivation, vomiting, a deep red or purple tongue, seizure and unconsciousness. A pet may suffer irreversible organ damage or die.
On a day that hits 85 degrees, the temperature inside a car with the windows opened slightly can reach 102 degrees within 10 minutes. After 30 minutes, the temperature will reach 120 degrees.
Source: Humane Society of the United States website