Some time previously on the now defunct photoshopper forum, a query was presented about "how do I check a used lens to see if it's any good?". The following from another contributor and myself were the answers which may be of general interest. Further experimentation led to a few enhancements to the information I provided in the second section.

Mount the lens on your camera and with the camera set on manual or Bulb, open the back and look through the lens and be
sure it closes down to all apertures smoothly and easily. Check surface elements. Minor cleaning smears are normal. So are
minor blemishes on the front element, these normally do not effect image quality if they are not particularly large and do not cut
through the lenses coating.

The rear element is very important. It should not be scratched and the coating should be fully intact. Some lens barrel looseness
is normal, particularly with modern, plastic barrelled lenses, but it should not seem excessive (this has to be a judgement based
on experience with similar lenses or other factors).

Screw heads that are marred or are rusted in the center indicate disassembly at some point.

External condition is an important indicator. Scratches that are applied from retrieving the lens from a bag over and over are
normal, but if the lens has evidence of impact, this could indicate problems. Many lenses survive falls, but you as a potential
buyer do not know to what extent the lens impacted, so you must insure the lens functions smoothly and properly when
mounted on a camera. A roll of film must be taken (at its widest aperture at a close object) to insure internal elements were not
jarred out of position.

Lastly, hold the lens to strong window light and carefully look through it from both ends. Though it can be difficult with some
multi-element lenses, look for hazing (possible fungus), excessive dust or particle matters on the inside. Anything you find that is
judged as a defect has to be weighed against all the factors combined, such as serviceability (if the lens is obscure and
technicians for it are hard to find and expensive), its general asking price relative to condition, etc. Many sellers do not closely
examine their equipment when trying to sell it and may legitimately not be aware of something you discover, some may know
and hope you take it before you see something you don't like, so each case is unique. Don't buy if you are not completely
happy, and always stipulate a period in which you are allowed to return in the event of dissatisfaction.... Anon

Further to Anon's advice.......

First make sure the external surfaces of the front and rear elements are very clean before trying to "look through" a lens to evaluate it. Of course remove any filters first. The best way to show the surface dirt is to use a strong torch (=flashlight in USA) aimed obliquely at the lens surface. All the dust and smears show up quite well. Puff them off with compressed air or squeeze puffer and clean with a lens pen or your favourite method until the oblique light test shows nothing on the outside surfaces.
Next put the lens front down on a light box (those nice little 5"x4" battery powered light boxes are good for this) with the aperture fully open to look through it for internal dust and fungus. Sometimes the dust and fungus shows up better if you use dark field viewing. Achieve that by sitting the lens cap on the light box and gradually raising/lowering the lens above it so the view is black but the light coming obliquely from the light box illuminates the internal dust and fungus well. If any lens elements can be rotated (front element focussing types) then do so to try and see where the dirt may be.
If your lens has a mechanical aperture open/close control lever, gently move it to and fro while the aperture ring is set to maximum (wide open). It should easily move with no sticking or hesitation at any point and allow the aperture to open and close smoothly. Check the aperture leaves for signs of oil. Some lenses, particularly if conditions remain hot, will leak lubricant from other areas to end up on the aperture blades and eventually cause them to stick and not open or close quickly.

Best not to do these tests with your own lenses as it is too depressing.... Guy