…my first guess would be that they keep track of the average yield per acre and if it is higher than expected (you don't see the adequate yield loss due to non BT acreage), then farmers were cheating (did not use the required refuge percentage).
The answer to the quiz is something MON is doing that not require any calculations to be made.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”