If they have no scratches on the lens, and no severe gouges on the body, then it's pretty difficult for there to be damage elsewhere.

One thing to look at, is whether the internal lubricants have leaked, smudging the internal glass elements and/or aperture blades. This was common with really old (1970-1980s) lenses, but I don't know if it still happens.

It comes about as a result of improper storage -- lens on its side, in hot conditions, rather than lens standing up in cool conditions.

The only real concern I have with used, recently made lenses, is that they may not be the sharpest ones available. Like I said, my own 70-300DO was hand picked by me from the three the dealer had on hand. One of the others was visibly worse when used on my own camera body (20D), and the third was probably equal to the one I bought.

Canon's quality control on highmedium-end lenses does not impress me (nor many other folk).

Cheers