1. Check that all the parts are securely fastened to the body, and that the neck is supported by a heel. If you want an acoustic guitar, make sure the soundboard is made of solid wood, not plywood. This gives the guitar a better sound.
2. Check that you can press the strings down easily all along the neck. If the strings are too high about the fingerboard they will be hard to press down. If the are too close to the fingerboard they may buzz when you play. You can have the height of the strings altered by a repaireman.
3. Hold the guitar up to the eye level. Close one eye, and look down the length of the neck towards the body. Check that all the frets are parallel.
4. Compare the pitch of the 12th fret harmonic of an string to the same note at 12th fret. If they are not identical, but the action is set correctly, you may need a guitar repairman to adjust the saddle or nut. This is difficult on a classical or acoustic guitar , but much simpler on an electric.
5. Play some notes on all the strings, all along the fingerboard. Think about whether you like the sound, how comfortable the guitar feels, and whether it stays in tune well.
6. If you want an electric guitar, check the sound when it is plugged into an amplifier. Check if the pick-ups and are control working.