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.
It's obviously not possible to control cryptocurrency flows and therefore tax them accordingly, but they can easily track every single dollar/fiat/whatever currency that's collaborating with an exchange, then see how much fiat you spent in a single currency.
When you want to cash out to buy real state or something, you will need records of all of your trades and crypto accounting, so try to keep track of that. I have lost some of my history in trading due dead exchanges and now im worried about that. This has happened to a lot of people and I wonder what they did about it.
Since I'm not a trader I do not have those problems but I have never stopped to think about that, those that traded in exchanges that have disappeared already are going to have a lot of problems proving exactly what kind of movements they did in there so it is likely you are going to have huge hurdles in the future when you have to prove where your money came from when you want to pay your taxes.