If you see something happen yourself, then you should call it out. You just saw them do something so you know they are guilty (or maybe they just did something to you!), so it is your responsibility to call them out immediately. Don't let it slide or wait for later or wait for the police. Right then, right there, do something.
If people do this as often as they see things happen, no matter how big or how small, then the problem people would soon become obvious to everyone. Each individual can decide for themselves who to believe, based on the full availability of the evidence, that is now right in the open for everyone to see.