[philbarr]

It's interesting you mention "beating". I've heard of, but not actually seen in action, a method where a piece of paper is placed on the strings with the guitar on a flat surface. If the guitar is strummed and the paper falls off it is out of tune, if it stays on it is in tune. Now you also need to hold certain notes down whilst you do this. I don't know what they are. And this might be a myth.

[SwellJoe]

I've never heard of this, but it might work on an acoustic guitar. Solid body electric guitars are sonically pretty much dead (maybe if you put the paper on top of the amp or a flat surface near the amp).

I just tested, and the results were inconclusive. Maybe I was doing it wrong, but there was nowhere I could put the paper that would result in movement when out of tune but not in tune. If I put the paper on the sound board with the guitar laying flat on its back, the paper didn't move much in either case; just a little bit of vibration/rattle. If I put it across the strings, it would fall off whether in tune or out (as I would expect, but I figured I'd try all the obvious options).

Perhaps I just don't know the proper chordal incantation, but I assume some combination of fourths and/or fifths or octaves, since those are the ones that will be very consonant/smooth/non-beaty when in tune and will beat very clearly when out of tune even a little bit.