If you go looking for problems, most people usually find some - real or imagined. If you do this test, you will most likely be disappointed no matter what you find. Poor centering is the most common issue with below par copies, or if a lens has been dropped, knocking the elements out of alignment. This shows up as softness in one or more corners.
Choose a distant target to minimise focusing errors, like a road sign or car number plate up the road. Lowest f/number, focal length somewhere near the wide end, and make sure the shutter speed is high enough to avoid all possibility of camera-shake. Push the ISO to get that if necessary, use image stabilisation. Focus very carefully, preferably in live-view, lock focus by switching to manual, then double-check focus. Lock exposure, eg use manual.
Position the target in each of the four corners, making sure that the target is exactly the same distance from the corner each time. Compare images. If you have a decent LCD, zooming in on that is fine. Do not expect dazzling sharpness, because you will not see that, even with the best lenses; what you are looking for is each image to be of a similar standard. A similar standard of un-sharpness if you like, this is a tough test. If you have to double check each image, there's almost certainly nothing wrong. Very few lenses are perfectly centred, so if there's a problem, it will be obvious.
