You can adjust the guitar neck by either loosening or tightening the truss rod. N ow check to see if the neck is properly fused to the body. You do this by grabbing the neck and the body of the guitar and then twist the opposite direction to see if there s any play at the joint. If you're looking at an acoustic guitar and it has play in the neck, put it back on the shelf. The cost of repair is probably not worth it.
Most electric guitars have bolt-on necks and can easily be fixed by a guitar technician.
But be sure to have this checked out before buying. The next thing you want to make sure of is that the acoustic guitar has a perfectly straight top. If there are bulges or the wood is shrunken in, this is a sign that the bracing is bad. Put the guitar up. It s not worth the hassle.