what i normally do when i buy a new lens is:
1. bring my own camera and test
2. check for dust in the lens
3. check on oil leaks
4. shoot at wide end and tele end of the zoom at maximum aperature, then check for picture sharpness.
5. step down one or two stops and do the same
6. check for focusing error
7. check for front or back focusing error
8. check for stiffness or looseness in the zoom
9. check for VR or AFS motor sound to make sure it is not too loud
10. check the M/A and M switch working as they should
11. shoot into the light and check for flare
12. check for corner sharpness
13. check for wear and tear (or signs of being used) on the exterior of lens
14. check the contacts are clean and make sure there are no scratches
15. check to see filter threads are not worn
16. lastly, ask if they allow exchange should the lens be a lemon.
that's usely what i do when i get a new lens that is expensive.
will take me about 1 hour to do the above, so far it has served me well.... but i would take extra caution when the review on a particular lens says it is a good lens with QC issues...
if time does not permit, then bring home and test....
as for testing the sharpness, shoot at various types of targets on a tripod and check that the lens is "sharp" as it should be....
if have friends having the same lens, borrow one and do a comparision of the same shot...
hope that helps....
