"Imagine you are a teacher and you suspect that some of your pupils have been copying each other's essays. When you challenge them they claim that they are innocent and the similarities only show that they all used the same sources for their research.
How would you prove beyond all doubt that one or more of your students had in fact been cheating?"
Okay, Cofty, I can tell you're really excited about this question. (It think it's the third time you've asked it.) I'll humor you if it makes you feel better.
So, a teacher? Okay. The pupils are cheating...hmm...Well, with all the lawsuits today, I don't think there's much a teacher could do if the pupils deny it.
Kind of a "pick your battles" situation.
What's your point? I know you really want to share it. Go ahead.