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.)
Quite a lot of work which almost nobody actually spends. If the gold funneling is supposed to create conflicts it is failing at it.
Players' fault? Perhaps, but if the feature fails to take into account how much OoC effort they'll tend to make I'd be more inclined to blame the feature.
