At the minimum, you would need an accurate volt meter, a test tone CD to measure single tone's voltage at the speaker terminal for each input and output which needs to be withing 1%; if not, the capacity to level them. SPL meter is not good enough.
So, off the bat, it is not easy. But, give it a try what you are able to do, have the wife randomly switch, or not, record which you think it is and at the end, compare your answers with hers; she also needs to record or have a random list prepared ahead of time for her switching.
9 of 10, 12 of 15 guesses will show something.