how do you know that the cashback the shop clerk is handing your right now for your diet coke purchase wasnt used three days ago for buying Heroin in the street ?
Because it comes from a known source. The problem isn't where the physical money has been in the past it's entire lifetime but what it enabled last. This is also why laundering money is a thing, because if you can prove you got the money in a clean way you are no longer on the hook.
We dont know and we dont care !