C-flat has the rights of it -- checking the scales, octaves and harmonics is a fast way to get a sense of how well the thing will play.. You want to listen for tone that pleases you, once you determine its accuracy. You don't want an action that is too high. (the degree the strings are raised above the fretboard, which increases as you go up the neck) such that you feel you have to work hard to make clean notes high up the neck. Action is usually adjustable, and what's acceptable depends on your muscle strength. If you don't have a lot of finger strength built up you might want to look for a good classic guitar with nylon strings. A guitar with an action that is too high can tire you out and interfere with the joy of playing.

Structurally look for something that has no body cracks. If it is second hand a few beltbuckles scratches in the back are inevitable but those are negligible. Even a lot of scratches in the pickguard area is not a bad thing necessarily, just meaning it has been played a lot, or perhaps very enthusiastically. The main thing is the accuracy of the fretting, the action, and the tone.

A