Stay calm. Most people who bring their dogs out for a day of car rides and adventure are dog lovers, too. Maybe they underestimated how hot it would get outside, or got held up in the store. Either way, there are some practical things you can do right now to make sure the dog is OK.
What to Look For
Not all dogs locked in cars are in immediate danger, especially if the car is on and the air conditioner is running. Before taking action, take a look (and listen) around the car. Can you hear the engine running? Air conditioners work by pulling power from the engine's belt —meaning the engine must be running for the AC to work. If you can't hear the car idling, that's a warning sign.
Some cars, including electric and high-end vehicles, run so quietly that it's difficult to hear the engine, even when the car is on. In those situations, you can press your fingers against the glass to see if it's still cool, but be warned, this method isn't fool-proof. If it's very hot outside and the car is in direct sunlight, the glass could be searing hot while the inside of the car is still very cool.
It goes without saying that the best way to know how the dog is doing is to look at the dog!
Dogs don't sweat. They can only cool themselves by panting and seeking out shade. When they're locked in a car, they can't escape the rising temperatures and quickly lose the ability to regulate their own body temperature — a condition also known as heat stroke. “If you see a dog in a car that is barking or appears otherwise alert and energetic there is a good chance he is not in any immediate danger.”
When dogs get too hot, their internal organs struggle to function and the dog becomes weak and confused. In just fifteen minutes dogs fall risk to permanent damage. If you see a dog in a car that is barking or appears otherwise alert and energetic there is a good chance he is not in any immediate danger. Even so the temperature inside the car can rise at a rate of 2 degrees per minute —so it is paramount to remain with the dog until the owner returns!
It's a good idea to get as much information as on the vehicle as you can. Take down the cars color, license plate number, and make and model to the best of your ability. Record the date, time of day and approximate location too, in case you want to report a crime later.
If you're in a shopping center or somewhere where there are other people, see if you can enlist the help of some other witnesses to wait with the dog while you go into the stores and page for the owner. Don't leave the dog unattended.
Breaking the Window
Is not recommended. In addition to hurting the dog when the glass shatters, there's a chance the dog will be very fearful, and will run or bite.
You can cool the car by up to 10 degrees by providing shade. If possible, parking your car nearby or shielding the other car with a towel or blanket will help ease the dogs discomfort. If the windows are open at all, you can provide give the dog water or ice until the owner arrives. If you're in a shopping center, there's a good chance that the dogs owner is inside — keep paging with the vehicle's details until they come to the car. A few minutes may feel like hours when a dog is in distress, so set a timer and a goal: “If the owner isn't here in 20 minutes, I'm calling the police.”
If you believe the dogs life is in danger right now, call the police. There are hefty fines for animal abuse and many cities now have a formal citation process which can include hundreds in fines, community service, and counseling. Take pictures and record clear, steady footage of the dog before help arrives to ensure you have enough evidence to guarantee the best outcome for the dog, and yourself.