I like older guitars. The wood they're using these days - well, let's just say the forests have been depleted.

The very first thing I do is sight down the neck to check for warping, twisting, cupping, and what have you. If the neck isn't straight, it doesn't matter how good it sounds. After that, assuming a good tune has already been applied, a few quick chords up and down the neck to see how she sounds, and mostly here I'm checking for an even volume balance so that I don't end up with real trebly open chords and real bassy barres at the ninth fret.

Never buy a guitar the first time you play it. Slap it around until they get sick of you and kick you out, then come back in a day or two and see if everything's the same. It's more important if you're like me and tend to use lighter-gauge strings than what the store puts on, and even then it can be a crap shoot. I picked up a Yamaha that was sweet as anti-freeze with a .011 E string, but move to .008 or 9 and it was thinner than Cher, very little on top and way too much bottom.