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.
I think that there is always that possibility of cryptocurrency transactions being imposed upon with taxes. It's a way for a government to control it, and they have all the power to actually do that . Cryptocurrency transactions are some of the most profitable transactions present today, and a government, which is always in search of business to help them gain more revenue, would surely not pass upon the opportunity to control cryptocurrencies, and one of those ways is through the imposition of taxes.