Puchasing a used lens, what to look for?

Besides the cosmetics of the lens, fungus, filter threads, what else should I look for.
Is there any on camera tests I can do? Older lens, to go on my D90, it is a nikkor
70-210mm F4. Any suggestions would be appreciated.

Michael

That's a great lens. You're gonna love it.
On camera tests: just make sure it focuses on the money. Look for creep. Hold the camera with the lens facing down. Does the barrel stay in one spot? If it doesn't, don't buy it.
Make sure there are NO scratches on ANY of the lenses, front and back. Clean the front lens properly. If there are any signs of dirt or anything inside the front lens, don't buy it.
Don't worry about the speed of the focus. That's entirely dependent on the motor in the camera or the onboard motor on an AF-S lens (which this one isn't).
I hope you get it for a good price. Let us know.

Jack, is there any way of checking the blades? I assume then that on camera I can set it to F32 and hit the
Depth of field preview button, but what about the other end? Want to make sure that none of the blades
are sticking or that there's no oil on them.

Michael

Michael, from my experience, there is no practical way, really, to get to the blades. The only way I know of to assure the blades aren't sticking (short of taking the lens apart) is to take pictures, and note the ease of operation. Blade sticking is an extremely rare and almost non existent phenomena. The lens would have to be pretty banged up for that to happen. Hope this helps in your decision.