Absolutely agree; the problem is how to prove it. You don't get to see players' transactions.
If secret police reports would reveal, say, the last 5 bank transactions, THAT would be useful, and cracking down on gold leeching would be much easier.
Their Lord and fellow knights know exactly how much gold they get each tax.
The Military Advisor can help you determine within a pretty reasonable estimate how much their unit costs per week.
The Secret Police can tell you how much gold on hand.
Do the math.
(No, I mean, literally: take the numbers there, do the math, and see whether they could be sending large sums home.)
