Since money is black, you may not detect the transfer. If some government can control all money flows, it can find all black money. So there should be some indirect measures. It is linked to estimation of the size of black or grey economy.

World Bank has some studies about such estimations for different countries. See for example: http://documents.worldbank.org/curated/en/311991468037132740/pdf/WPS5356.pdf

So much thank you Yuri Yegorov

https://www.thehindubusinessline.com/multimedia/archive/01091/WhitePaper_BackMon_1091518a.pdf