Once you're inside a guitar store, there is a distinct protocol that should be followed. Here are some tips:
After spotting a guitar you like, walk up to a sales rep and tell him you'd like to play it.
If you know nothing about guitars, try to get a friend who does to come with you (or at least have someone show you a few chords to play). Do not let the sales rep play for you, because he will make everything sound good. No matter how bad you are, you should be the one playing it.
Likewise, don't let the salesperson plug your guitar into some monster amp, because an amp will make even the shabbiest of guitars sound good. Trust your ear; you know what garbage sounds like.
Always comparison-shop. Listen to a guitar that's way out of your price range and work your way down to the worst. That way, you'll know exactly what you're paying for and what you're missing.
If you think the sound is good, do a quick check for "fret buzzing." A fret buzz occurs when the wood is warped. When you pluck a fretted note, it should sound clean. If you hear a buzzing or weird vibration, send the guitar back and ask for a new one. Also test the harmonics and octaves around the fretboard to make sure everything sounds in key.
Once you've found a guitar you like, talk to the salesperson and make sure that you can get valuable services with your purchase, like a warranty, or maybe a free lesson or some wood polish.
With the deal sealed, buy the guitar. Now all you have to do is learn how to play.
