There are already large numbers of treaties in place for countries to cooperate in tracking down tax evasion. It might not be as expensive as you think. If you think of it from a practical standpoint, we have lists of the richest people in the world. That’s an excellent starting point, isn’t it?

If some billionaire is claiming that they actually aren’t a billionaire, and that the lists are wrong, when some government is trying to tax them at an exorbitant rate, it’s likely that they will give all of their banking details to said government to prove it. Or they will hide those banking details, and they’ll be forced to pay the taxes.