I found a nice checklist online quite some time ago and have it saved on my computer (for both bodies and lenses).
Hope this helps! If you would like a PDF copy of this, feel free to send me a PM.
Lens Checklist
1. Look at the front and rear lens elements. Are there any scratches? Angle the glass towards a light and look at the reflection. Are there any marks or swirls in the lens’ coating?
2. This one is kind of up for debate, but hold the lens towards a light source and look through it. Is there dust or fungus on any of the inner elements? While some dust is normal on almost all lenses, fungus is not. Fungus is a deal breaker for me because it etches the glass and can never be fully removed without a re-polishing. Dust on the other hand isn’t something I worry about unless the seller said the lens had just had a fresh cleaning or is supposed to be new. A film can also be present on older lenses. If it’s supposed to be a clean lens, make sure there isn’t a lightly frosted look to the inner glass as it will affect the image quality.
3. Check the lens mount for brassing. While brassing isn’t a deal killer, again, if the seller said in like new condition, be sure that it is.
4. Check the lens terminals. Make sure the pins aren’t loose and that it doesn’t look like someone did a crappy repair job.
5. Mount the lens to the camera. Check to see that there’s very little play between the camera mount and lens mount. Also make sure the camera doesn’t show any kind of error message when moving the lens and holding the shutter half way down.
6. Does the lens AF? Make sure it does.
7. Does the lens focus to infinity and up close through the entire zoom range.
8. If the lens has a focus scale, check for cracks in the plastic. Then check that the scale matches what the lens is doing (i.e. if you’re focused to infinity be sure the scale shows infinity).
9. Use AF and select a focus point; take a picture. Review the picture and check focus. If your camera works with every other lens you’ve mounted on it, but not with this one; there’s a problem.
10. Use manual focus and be sure it’s smooth; if it catches, there could be a problem. The same goes for the zoom; If it catches, there could be a problem.
11. Check that the filter threads have no flat spots and that a filter will screw into them
12. If the lens has IS and other switches, be sure these features work. On most image stabilized lenses, you can hear the IS motor working.
13. If the lens is supposed to have full time manual focus override, be sure that it works.
14. Set the lens to infinity focus and focus on something up close. Does the lens AF as fast as it’s supposed to? Different lenses will have different focusing speeds, but knowing how slow or fast it should be is important. If it’s horribly slow and it’s supposed to be lightning quick, there’s a problem.
15. Check the lens grips and be sure they’re snug. While loose grips are pretty common on older lenses, they are normally cheap and easy to replace. If it’s a known problem before you buy the lens, be sure that you can get a replacement and that they’re not discontinued. While this isn’t a huge deal, I’d hate to use rubber bands on a lens I just paid through the nose for.
16. Check the outer condition of the lens. If the lens is supposed to be new, check that the lettering isn’t starting to wear off and that there aren’t scuffs in the paint.
17. To get real picky, look at the screws that hold the lens together. Professionals use the correct screw driver sizes so that there’s very little damage to the screw heads. If the lens screws are all mauled up or mis-matched, it might make me think twice. If it’s a new lens, then that’s a no go.
18. Do a shake test. No, I mean give the lens a little shake. Does anything rattle? If it does, what is it and where is it?