With the amount of BTC that passed through silk road and the likes, I think only newly mined coins can be called clean.
I remember people paying a premium for such newly mined coins.
Wow, how does it feel coming back to the forum after 9 years?
There is no such thing as dirty coins, it's all a made up story that can possibly target the most vulnerable ones, who fear that governments will hunt them down because they have a wallet with some coins in it.
There is no way to tell if a coin is "dirty", because to define dirty you must first censor some addresses. But who is responsible for censoring addresses since Bitcoin isn't authorized by any individual or entity?!
It's all a game of power. They just invent things to try and control bitcoin because they can't control it otherwise.