Scratches in the glass.
Mold or Fungus inside the lens.
A little bit of dust inside the lens is normal but too much and it may require a professional cleaning.
Excessive wear & tear on the body of the lens.
Check that the focus/zoom rings work smoothly.
You will want to make sure the lenses focuses properly, and that if it's an AF lens...that the AF works. Same goes for IS/VR etc.
Here's one to find if fungus has ever been on the surface of the front or back elements. Breath on it. Fungus etches itself into glass if left long enough and while the lens will look spotless misting the glass will show where it has been affected.
That said this is not an indicator of a bad lens, but more a talking point when asking the person why he mistreated his equipment and is charging more than you are willing to pay