A flashlight test is useful, but it can be misleading: even excellent lenses can show lots of internal dust or tiny marks under strong light. What matters is whether defects affect images.

General guide:

• Internal dust: usually normal and rarely visible in photos.
• Minor scratches: often less serious than they look, especially if image quality seems normal.
• Fungus or haze: avoid it; this is a major warning sign.
• Front-element scratches: may not ruin photos, but they can indicate poor treatment, so buy only with caution or a discount.
• Mechanical condition: very important. Focus and zoom rings should move smoothly, and the lens should not feel loose, gritty, or damaged.

If you can test the lens at all, take a photo of an evenly lit plain surface at the smallest aperture to help reveal issues, much like checking for sensor dust. Also check autofocus/focus accuracy if possible, since impact damage may not be obvious from the glass alone.

Beyond inspection, buy from reputable sellers and compare the asking price with typical used-market prices. A price far below normal can be a red flag for hidden problems.