[YendorLootmonkey]

I always figured the server kept track of roughly how much currency was in the game at any given time. Considering currency only enters the game via quests, vendor sales, or mob drops, and only leaves the game via destroying coins, vendor purchases, or quest turn-ins (trading stuff for plat doesn't impact the total currency in game, it just shifts the distribution of it across characters, so you wouldn't need to track this)... it should be easy to track this "rate of net currency increase" in the game and what the average rate is.

Have red flags go off when the net currency increase jumps way beyond that rate within a given time period. That would give you a pretty good indicator that currency entered the game via a dupe, because it seemingly came out of nowhere. Or if particular mobs were dropping an abnormal amount of plat after a patch.