We have a program in Canada, the CRA has, for doing what's called a net worth audit. You take a look at the net worth at the beginning of a period, the net worth at the end of the period, calculate what living expenses are, and see what you've declared. It's a fairly effective program.
But as long as people can spend cash.... I think the biggest deterrent would be to deter the ability to spend cash.
And transporting cash across borders is becoming difficult. Using ATMs is becoming difficult.