How to accuse a tenant of a breach in lease?

[Ralph Pena]

I'm just curious as to how do you go about notifying and penalizing a tenant of a breach in contract.  Such as smoking inside property,  or having a unauthorized pet and etc... IF YOU HAVEN'T SEEN THEM WITH YOUR OWN EYES? ?? 

If you see it with your eyes fine. But what if neighbor reports it or someone else but can't be proven.  Any help is appreciated guys.