Great thread ddw. One of the best way I know for a last second "quick tune" is to play a chord with only "E"s and "B"s in it. Here's how it works: (wish I had a guitar in my hand) with your index finger play the 7th fret 5th string (E). the 9th fret 4th string (B) with your middle finger, and the 3rd string 9th fret (E) with your ring finger. Strum all six strings hard. If you're perfectly in tune you'll hear this really resonant (partial) E chord. If you're not in tune (even slightly) it'll sound awful!

Try this, it works.

Rick