Hi Jswk,
Here are my thoughts (on a night that I am suppose to be working but unable to because of a clouded mind).
1) Set your expectations and Source for guitars that meet up to them; if you're expecting a 'nothing less than mint condition' guitar but go round scouting for big brand names without clarifying the use and abuse the guitar has gone through could be fatal. If you're expecting a cheap deal for cheap thrill of trying your luck, then it will very much be that; Meet up at an MRT station, handle the guitar for 5 minutes, pay up and go... In the right hands, cheapos can be a raw Gem waiting to be polished.
2) If you're going for quality, then check for quality. Everything and more as described by earlier posts. If you're going for a no frills bargain, manage your expectation.
3) The worth of the guitar is the sound that wil eminate from it from which you can create your music with, so make sure that it sounds the way you'd expect a guitar to sound (if acoustic, it is easy to hear the sound of that strum out of the box, if electric... got to figure out a way to test if dealing at home is a big No! No!
Unless of course you're buying for display, a collector's item or a modern day antique that is intended more as a conversation piece or future heirloom.
Good luck!