Sonically, the three things you should really listen for is:
Balance of the strings- you dont want the EAD strings drowning out the bottom strings. You want your fingers to be the only thing that controls the relative volume.
Overall volume - really the louder the better
resonance - how long will it hold the note, you'd like it to hold the note as long as possible.
Physically, look for cracks of course, make sure the neck is solidly attached. If you want a brighter sound go for a spruce bodied guitar. For a deeper smoother tone, you want a body made out of cedar. I prefer cedars, but its really subjective.
Find the ones in the general price range range but then try not to pay attention to price because there is a large variance between quality and price for that price range. In other words, you can find a guitar that sounds better that may be 50-100 dollars cheaper, let your ear guide you.