I know that Farmville was never designed as a game for learning. But when my youngest kid, at 5 years old, learned how to calculate percentages on time to harvest on time, I realized he had learned.
So, what I mean is, I think games can teach and the best way to know if they taught well is to discover what the kids know. In most cases the game is an accessory a teacher might use. The teacher should be able to tell if the students' knowledge has improved.
I am sure there are tests too, but observation will give you the first results.