J.B.
Well, if you don't actually see anything happen, you can't say anything, but if I see something happen, I always say to the other kid, "Stop (hitting, kicking,whatever), or It's not okay to ________ or We don't do________here." I say it sternly and firmly -never yelling. Kids will usually go away at that point or completely back down when they realize they've been called on something they know they're not supposed to do -especially by an adult stranger. I have no problem letting any parents who suddenly materialize and take issue that their child was doing whatever it was.