The usePowerOf2Sizes is set as a bitwise "user flag" on the collection, so it appears in db.collection.stats() but isn't obviously labelled at the moment.

As of MongoDB 2.2 this is the only collection-level user flag, so you should see the userFlags set to a value of 1 if enabled: http://docs.mongodb.org/manual/reference/collection-statistics/#collStats.userFlags