To answer this question we would need net money flows in and out of exchanges.

Even in the financial markets people tend to take ETF net flows as a basis of estimation.
But there at least there is IPO data and top holdings data from which you can fairly well calculate net new money into a stock.

I think using forex market multiples maybe a better approach than saying nothing, but there are so huge differences in long term and short term market participant ratios from market to market that it is fairly unuseful. Take for example huobi's 0 fees, that alone makes forex trading volume uncomparable to Bitcoin's