As for what to look for.
Make a physical examination of the lens. Some braising on the barrel is normal on older lenses. However, paint chips or bent filter threads might be an indication of hard use. Look at the front and rear elements. some scratches on the front element is not a major cause for concern. Scratches on the real element are problematic. Look at the lens coatings on the front element, it should be even across the whole glass.
Next shine a light through the lens and look through it. Fogging and fungus are not good signs. Some dust specks inside the lens is normal.
Put the lens up to your ear and listen carefully as you move the focus, and zoom rings is appropriate, through their whole range of motion. If you hear a bit of grinding then put it down and walk away. Also look for smoothness of motion as you do this. The focus and zoom should not have any areas where you must significantly change the force necessary to complete the motion.
Check for any slop in the lens barrel when it is extended through zoom or focus. Some of the less expensive plastic lenses and some of the super zooms will tend to develop a bit of play over time.
Look at the mount. Any bent bits or missing screws is a good enough reason to leave it with its current owner.
Mount it on your camera and take some photos. At this point all should function properly. No stuck aperture blades, consistent results at different apertures and at repeated aperture settings. Look at the image on a decent monitor if you can. This is the time to pixel peep. If the aberrations are within the expected range for the lens then you should be good to go.
Cosmetics aside, a second hand lens should be mechanically flawless before considering making an offer.
Some really excellent lenses can be had on the second hand market and sometimes veritable steals if you can see past the cosmetics.