This can be a tough call; first we must look in a mirror and think about what our children have done and we must also recall how we "screwed up" in parenting at times.
Of course it would all depend on the circumstance, but I usually find that confronting with a smile and with love is the first key. Usually telling how we handled our kids through similar incidents always keeping a firm grip on the parent's attitude during the encounter.
I must say, be careful in doing this, because some adults are worse than kids and will be vengeful. There are people that can be called too. Sometimes allowing a kid to run rampant is a form of child abuse and there are many out there who work towards stopping abuse.
I, personally, am a big fan of BACA--Bikers Against Child Abuse and even wrote a hub promoting this great organization.