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.
Bitcoin is just like other modes of investments such as real estate, stock market but the difference lies in the digital nature of the bitcoin. Besides the potential of bitcoin to fluctuate in prices at any time and at any level. So if you have invested your hard earned money into bitcoin and are now enjoying the sweet fruits of your investments, I dont think so there should be any problem in proving your assets to state.