About to buy a used lens,...

I am thinking of buying 70-200mm f/2.8 IS. How do I check if the IS is working or not? Can someone help me with this?

Turn it on and shoot at a slower shutter speed. You'll also hear the IS turn on and off as you start focusing.

Ok thanks man! I will do that.

Also, you can see the results in the viewfinder as well...especially at the long end. Zoom out and look through the viewfinder while moving the lens slightly. Make sure the IS isn't on...then activate it. You should be able to see that the image in the viewfinder is more stable...it may appear to 'jump' from place to place as you move the lens.

Alright, thanks.. What else do I need to check? Can you advice me the things I should do when I inspect the lens?

Buying used lenses is a great way to save money and get better gear than you normally would. All but two of my lenses have been bought used with nary a problem.

Always make sure that you review the lens on-line.

When you first meet with the seller, look at the lens carefully. What is the condition of the exterior? Is is scraped or scratched? Is the filter-ring damaged (always ensure you know what diameter filter the lens takes and bring a filter with you). If a filter won't screw on easily, question the seller, it's usually a good indication that the lens has been dropped or banged hard.

Ensure you have a small penlight and look through the lens from both ends; look for any odd refractions which might indication displacement or separation of internal elements or fungus growing inside. A little internal dust is nothing to worry about. Hold the light against each end element at various angles to look for cleaning damage, scuffs, swirl-marks etc. Again, I would walk away if there are any; this is an indication someone didn't know how to care for the lens.

Close the aperture and watch it carefully. When you let the pin go, it should snap back quickly and evenly; look for any signs of oil or residue on the blades. When they're closed, do they form an even pattern, or do one or more appear to not be in the same position? If so, you know what to do here.

Smell the lens carefully. Sound funny? Probably, BUT if a lens has been stored somewhere damp you may smell a damp, moldy/mildew smell.

Mount the lens on your camera; ensure that it functions correctly in ALL modes, take and CHECK test shots in manual mode at all apertures. Mount and unmount the lens several times, ensuring that it feels smooth and doesn't catch. Manually focus the lens and operate the aperture ring (if so fitted) if it's a zoom, operate that. These functions should feel smooth and positive. If there's slop, grinding, or points at which something feels wrong, walk away. Ensure you do this with the camera in normal shooting position, upside down, lens pointed up, down, etc. often a problem can manifest itself in one position and not in others.

Thank you so much. I would have never thought to do anything you mention...especially smelling the lens.