I'd spread the word to other teenagers asking who it was.. A hefty reward would be given and some hands would get broken with the rock(s) that were thrown..
A local tow guy down here just painted his tow truck.. Some kids "tagged" it with they're name.. Lets just say the kids learned a valuable lesson that will stick with them till they die...
My pops had a Mango fruit thrown at his windshield passing a high school going 60mph.. Didn't catch the kid but if it was any higher it'd have been a serious accident..