You usually can’t reliably detect every minor problem without visible symptoms, but a few simple checks help.

Inspect for obvious signs first: dents, misalignment, loose parts, scratches, or chips. Keep in mind that front-element damage may have little visible effect, while rear-element damage is often much more noticeable.

Then make test photos:

• Shoot a plain bright subject like the sky or a white wall.
• Use small apertures (around f/8 to f/16 or smaller), where dust, chips, and scratches tend to show up more clearly.
• Look for dark spots, repeated marks, banding, poor focus, or uneven sharpness across the frame.

To isolate the problem:

• If the same issue appears with different lenses on one camera body, the problem is likely in the body/sensor.
• If the same issue appears with one lens on different bodies, the lens is likely the cause.

Also note: dust inside a lens or body may have little or no visible effect, so not every internal speck means damage. And some dropped cameras may show no obvious signs unless they cause focusing or image-quality problems. If buying used, trust in the seller still matters.