There's no way on knowing about that and you're right then, most likely and in general all coins has its source for being tainted. So, in summary, no matter where it comes from, there's a trace of it from being tainted.
Exactly my point. The whole thing is completely arbitrary. Let's say we have some coins which we know were stolen, so we taint them. Some of those coins then move to another wallet. Are they still tainted? How do we know that the coins weren't just spent in a completely legal way, such as buying goods from a merchant? Now we are punishing the merchant for a crime they didn't even know about, let alone have anything to do with. What about after the coins have moved 10 times? What about 100 times? Are they tainted forever, or do we just pluck a random number out of thin air and declare that they are clean again after this many transactions? What about if those tainted coins are combined with some clean coins? Are all the outputs tainted? What if 0.01 tainted BTC is combined with 0.99 clean BTC. Is the full 1 BTC output tainted? Is it 1% tainted? What does that even mean? What about when a centralized exchange accepts those tainted coins? Are they magically scrubbed clean again?
Taint is completely arbitrary made up nonsense. The best option for both the individual and the wider bitcoin ecosystem is to not do business with anyone who forces you to obey their made up rules, since doing so defeats the very point of a decentralized, peer to peer currency.