I think it is not realistic to be deaf in the country for every citizen, how much he owns crypto-currencies. Yes, and still recover the tax from it. To be able to collect taxes, the state should control the system itself, where the crypto currency is actually used - and this is not realistic for a single country. Scales are too huge.
When you need to money out to purchase genuine state or something, you will require records of the greater part of your exchanges and crypto bookkeeping, so attempt to monitor that. I have lost a portion of my history in exchanging due dead trades and now I'm stressed over that. This has happened to many individuals and I ponder what they did about it.
It's clearly unrealistic to control cryptographic money streams and in this way assess them likewise, yet they can undoubtedly track each and every dollar/fiat/whatever cash that is teaming up with a trade, at that point perceive how much fiat you spent in a solitary money.