The practical way to check is to shoot test images and compare sharpness and behavior across the frame.

Start by making sure the zoom and focus operate normally: no unusual noises, hesitation, or rough movement. Then take photos at both the wide and tele ends of the zoom, and also at a distant subject (near infinity focus), since problems may show there first.

For image quality, photograph something flat and detailed straight-on, such as graph paper, a brick wall, or another repeating pattern. Review the images on a computer at high magnification and compare the center, edges, and corners. A common impact issue is decentering, where one side or corner becomes noticeably softer than the rest.

Some variation across the frame is normal, so look for obvious unevenness, blur, or distortion that wasn’t there before. If you have a scene you’ve photographed often, shooting it again can also help you spot changes.

If everything looks sharp, the zoom works normally, and you don’t see uneven softness or distortion, it’s probably fine. If you want a definitive check, a manufacturer/service inspection is the only sure option.