Can you find out when a person is guilty of some wrong?

You do the right thing by not pointing others mistakes so long if it doesn't harm others.But if necessary one can gently hint at the mistake commited and then quickly switch over to another topic.If the mistake is unintentional and harmless then I would better ignore it. As the shloka says.....apriyam satyam na bruyaat(don't speak distasteful truth).

Yes it is very easy to find out if someone is guilty, since such people behave ackwardly, they are afraid and confused. The facial expression of such people tell everything because the guilt of a guilty man can be seen in his eyes. If you are sure that certain person is guilty then you should talk to him, try to make him realise his mistake through decent conversation, if that does not help.Then you have full right to take necessary steps. Just be sure that you don't bring any harm to yourself. Today everything is possible .