Bitcoin is decentralised and anonymous so how can governments issue /enforce taxes on any transactions made with bitcoin?
Governments can set up laws and regulations and then set up "departments" to monitor and enforce.
Whether it is successful or not is another question. No doubt they will have to skill up and understand how digital currencies really work, maybe monitor a few people, monitor the flow of their btc's and audit them, create some "high profile" cases to be made examples of etc.
One can draw an analogy with cash.