Bitcoins itself could not be taxed because it is not regulated by central banks,it is a virtual currency working on its own. Unlike fiat money which is governed by the rules in the central banks and government, but even though its is not taxable, the way we spend our earnings is already being taxed. Like buying goods and services, taxes are already imposed in everything we pay.
Well if your government says they are taxable, then it can be regulated and taxed like in the US. The government can require exchanges to send user trade data at some point in the future. When you cash out to fiat the gov will want to know the source.