I don't advocate putting a screw driver to a gun unless it belongs to you and even then it's 'screw at your own risk'.

When I purchase a used shotgun it must first make it past the visual test, naked eye and magnified.
Then the 'feel' test.

If the gun has made it past those two then I'm going to measure the bores.
Once it's determined that it's passed and I'm 99% sure it's a keeper I probably will shoot the gun.

I've sent several guns back but none that I ever shot.