I was in a similar situation except the stupid lady backed into my car.
I parked behind her, and left the car. I heard a big bang and turned around. She had backed her car into my car even though there was no car in front of her. So she did not need to back up at all. Dumb woman probably put it in the wrong gear.
I saw no apparent damage because she hit my car on the license holder. But then a few weeks later I realized that the front bumper was loose. She must have broken some of the holders behind the bumper.
Lesson? Just because you can't see it does not mean there's no damage. However, if you push on the rear bumper and nothing moves, then you should be ok. Even if it moves a little bit, you probably don't need to spend money to fix it.
Also, I hate these goddam careless drivers. Wish them all cancer.