To see if it's optically a good copy with all elements correctly centred, shoot a distant target (street sign, car number plate, road sign etc) to minimise focus errors, at lowest f/number and a fast enough shutter speed to guarantee zero camera shake.
Lock focus and exposure on manual so it can't change and take pictures of the target in all four corners of the frame. Take them quickly because if the light changes it can have a big effect on the result. View them on the LCD at maximum magnification. They should all be equally sharp, or possible equally unsharp as this is a very tough test. But equal.
If one or more corners is noticeably worse than the others, then something is out inside - maybe after a knock.
Testing for ultimate sharpness is extremely difficult unless you have another lens of known good quality that you can shoot an identical subject with, side by side. with identical settings.