Purchasing second hand lens

[csgrafix]

I'm thinking of purchasing a second hand Canon EF IS L series zoom lens. I will have the ability to see/test the lens before purchase. What should I do to test lens and be sure its working properly ?
Any advise on what I should know/or do before buying it ?

 

[bazooka]

I'm no tech guru, but here's a few for starters.

Mount it to YOUR camera (with sellers permission).
Check the IS turned on. Turn IS off and make sure it stays off.
Check manual focus...ring should turn smoothly, you should feel no dirt.
Check auto focus between far and near several times (good light preferably). Notice accuracy.
Check zoom control for smoothness.
Check for abnormal softness or aberation and both ends of the focal length, especially around the edges of the frame.
Use a light and check both the front and back lens elements for scratches or pits. If it has anything questionable, you might ask if they would clean it a bit for you... that maybe too nosy, but it's your call. Some pits/scratches may look like sap, vice versa.
Check contacts for corrosion.
Offer low-ball price.
Buy.

 

[csgrafix]

Check the IS turned on. Turn IS off and make sure it stays off.

Not sure if I know what you mean by this...
How would I know if IS is actually working ?

Check for abnormal softness or aberation and both ends of the focal length, especially around the edges of the frame.

Do I inspect picture taken zoomed in on the camera's LCD ?

Thanks for your reply.