Dogs Today Magazine, in the UK, has a great site detailing information about how hot a car gets on a warm day, windows open or not. Granted, the car experiment they did records the temperature results in Celsius, not Fahrenheit, but Google is your friend when making temperature conversions. And actually, in their very last picture, you can see that even with all four car windows open, in the sun, the interior reached 50 Celsius, which looks remarkably close to 120 degrees Fahrenheit.
According to PetMd, some signs of heat stroke are: panting, rapid heart beat, irregular heart beats, reddened gums and moist tissues of the body, excessive drooling, seizures, and shock. There are more, but those are some apparent ones, and you can see the whole list on that site, if you'd like.
One thing I've noticed about my area is that there is hardly any shade to be found in parking areas. Really, I rarely have Elka in the car with me anyway, because she is permitted in so few establishments, I figure she's just happier at home.