If not, then the only method I can think of is that crypto enthusiast will have to report it to their local tax authorities. Now, it's just the question of morality, are you going to report your personal crypto profits or not? Are you willing to take the risk and hide before the authorities have to find it out?
That's exactly what I was thinking. How can they tax a decentralized crypto? There is no way one can trace the ID of person associated with a particular wallet i.e BTC is Pseudo-anonymous. And that's the ideology of BTC and that's why govts are mostly against the crypto since they cant tax it.
The point is crypto that comes via centralized exchanges is not what Satoshi has created, don't call it cryptocurrency rather its just digital currency.