Otherwise referred to as The Foreign Women Litmus Test.
I like to sing famous songs in the shower but insert my own lyrics.
My favorite is Riders on the Storm but inserting the words of Children of the Corn in place of The Doors’.
If you get out of the shower and the room is empty, turns out they understand English better than you thought they did.
9 times out of 10 though, when I get out of the shower, they are lying naked on the bed looking at their cell phones, completely oblivious.