There are three steps:
1. Take information. Note the make and model of the car, the license number, exact location, and a description of the dog: breed, colour, size. etc. Remember that windows cracked open do not significantly reduce the internal temperature of a car.
2. Assess the situation. How long have you been present while the dog has been in the car? Is the dog in distress? Signs of heat distress include excessive panting with the tongue fully extended, stumbling, glazed eyes, disorientation, hiding in the footwell, and ultimately, coma and death.
3. Notify authorities. Contact nearby stores and businesses. Ask them to make an announcement for the owner to return to their car. If the dog is in distress, call 911. Stay on the scene to monitor the situation until the owner or help arrives.
And of course, don't be a part of the problem. Don't leave your pet in a hot car.