If you became rich directly from your investments in BTC then that is an easy thing to prove, and you should not have any problems with any government. On your capital gain you should pay taxes as well. The same way you would if you became rich from real estate or playing the stock market.
I believe that everything can be proven, and what that has with anything, people know are they doing something legal or illegal and if they get caught there will be some consequences.
To add a little humor into this gray topic, do you know what you never ask a woman? How old is she! And what you think what you never ask a man? How he earned first million! How older I am, more I believe in this saying, it is like that. Earn that money somehow and if you continue to earn more its ok, you will cover your tracks, and because you are so rich nobody will ask you nothing, but if you don't continue to earn and make more profit government will bust you, one way or another, legal or illegal you need to pay some kind of task and protection from governments.