1. Check that it is the len you want (mk I instead of mk II etc)
2. Make sure it works as it should be (AF, USM, IS)
3. Make sure what the seller says (mint, 10/10, good optics) is what the product is
4. Call up the seller to confirm the quality of the optic (any dust, scratch, faint scratch etc), exterior condition and smoothness of the zoom
5. Look through the len under a light source to look out for any dust peck, zooming in and out in the process
6. Mount it and take it with your digital camera, zoom in and see the quality of pictures. For film users use a roll of slide and process it at RGB (what is $5 compared to hundreds to thousands of dollars you going to spend). Let the seller know first before you going to go by the slide way.
7. Last of all, go to a shop to test out the real len new before you buy it 2nd hand, therefore able to compare it with the 2nd hand len whether if the zoom or focus ring is too loose etc.