Interesting. Thanks for posting, f-troupe.

I don't think that I listen for beats when I tune. I listen to the pitch (is it high or low?) I've discovered that when a string is a tiny bit high or low (sharp or flat) I can't tell what's wrong by listening. But if I hit the first string and sing its pitch, then hit the second and sing its pitch, then the muscles in my throat tell me whether the second one is sharp or flat.

But mostly I rely on a "black body" (electronic tuner) to do it for me. Once the instrument is tuned, my cat, Schroedinger, comes and listens.