Ask the owner to tune the guitar
Guitars that are poorly-constructed, in bad-condition, and or old may have a hard time holding their tune. Do this step first so you can check the status of the tuning after a thorough examination.
Verify the guitar maker and model
As is tradition with classical and acoustic guitars, the maker (or in modern times, the company) places their name along with a model number on a parchment found inside the body of the guitar. This should be visible from the sound hole.
Look for cracks in the body
Cracks in the body are a sign of mistreatment and negligence. Dropping a guitar or poor humidification can cause cracks in the body.
Look down the neck of the guitar
Have the headstock or bridge of the guitar in front of your eyes and look down the neck. The neck should be straight and not bowed in or out.
Examine where the neck meets the body
Turn the guitar over to the back and examine where the neck meets the body. Cracking or a glue job here is a major warning.
Solid Body Electric Guitars: check for rust
History of the guitar
Ask about the history of the guitar. Focus on the age, usage, and storage conditions. It is best if the garage was stored in a case or properly humidified room. Garages, attics, or next to vents are not ideal locations.
Have the owner play the guitar
If the owner played the guitar after the tuning, have them play the guitar again.The guitar should still be at the same tuning as before. Guitars in moderate and constant temperatures should stay in tune for hours, not minutes. After a day or two they should only need minor adjustments. Also listen for buzzing, rattling, and any other undesirable noise. There is no such thing as over playing the guitar. If the owner will not play the guitar or allow you to, do not purchase the guitar!
Have the intended recipient play the guitar
Have the intended recipient play the guitar. Make sure the guitar is a good size for the player.
Take time to make a decision
Explore your options. Do not feel pressured to make a purchase right then. If you find the guitar needs work, please consider the value of the guitar before purchasing. If the owner is selling you the guitar for $50 but the guitar requires $50-75 dollars worth of parts and labor, consider purchasing a new guitar. It doesn’t matter if the owner claims to have purchased the guitar for $200. You can purchase a brand new, decent classical guitar for a student for $125 and up and an acoustic guitar for $100 and up.