Here's a list of offhand questions which may help:
Before your purchase, determine your
1. Budget
2. Folk or classical
3. Type of tone you're looking for
4. Full body or a single cutaway for easier access to high notes
5. Whether you want to plug it into an acoustic amp
6. Research on the various brands
During purchase,
1. Bring a knowledgeable friend along
2. Choose a time to test out when the shop is quiet so you can hear what you're playing
3. Don't be afraid to try out the high end guitars so you get a gauge and feel of the difference in high vs. mid vs. low end stuff
4. Usually you have to purchase your own case but can try bargaining it in as a freebie
5. Shop around as much as you can. Don't make the purchase on the same day, sleep on it and if a particular guitar still haunts you in your dreams then that's the right one.
For folk guitars, I for one recommend the brands of Yamaha and Cort as excellent value for money.
When testing acoustic guitars, I take important note of the action, playability (different sizes of necks available), workmanship, loudness and tone. Just play something familiar, pick a few notes and strum a few chords and see how much you like it 8)