First off, I'd get some digital photos quick.
Then I'd leave a nicely worded note along the lines of 'I found a new dent in my new car that looks like it matches up to your car. Could we discuss it?'

Be cool about it. MOST people can be reasonable. If there's no paint damage, as mentioned already it shouldn't cost more than $100 to have the dent removed. Below either's insurance deductible for sure. If you discuss it rationally (i.e. don't start with 'look what you did to my car!'), chances are this person will pay for the repair.