If something looks tired then it probably is.
You get some pro gear which has been used as a tool, banged about, used every day in all weathers, perhaps for 100's of shots each day.
With ring type USM's there shouldn't be much of a wear problem as they aren't conventionally geared, and the FTM mode means there wouln't be any crunching during use..
I would be more concerned with the condition of the aperture. A heavily used lens may have worn blades, and oily diaphragm etc.. pop it on a body (pref not your own) and look through it as you stop down and press the dof preview button.
It should stop down immediately and release immediately. If not. Walk away.
The iris shape should be regular, at all apertures. If not, walk away.
The iris blades should be clean and free from smearing. If not, walk away.
Other things to look for are dings in the filter thread (take any filters off and inspect.. a sure sign that the lens has been dropped) if theres a ding walk away.
Check the zoom ring turns smooth from end to end. Switch the lens to MF and check the same.
Look through the lens for anything in the glass. A little dust is fine (although you wouldn't expect so much in a weather sealed lens) beware anything that looks like a fine spiders web, or splodges of white. This is most probably mould, so walk away very quickly and wash your hands.
A little bit of paint chipping is fine and normal. If it's excessive then it's had a hard life.
The paint on the bayonet mount for the hood will no doubt be a little worn, and the black plastic hoods scuff if you look at them the wrong way.
Check the coatings, very light scratches are ok (these will be on the surface, internal 'scratches' are more probably mould) chips or major scoring suggests an abused lens.
One other issue is parts. If you buy an older used lens then there may be a service required at some point. Try and glean if the lens is still in production, and if not, if parts are available. Call a canon service centre for advice.
I see lots of very nice looking lenses at good prices on ebay which have seemingly minor faults, but are major if you cannot get the parts.
Check return policies and warranty periods. For expensive kit you should expect a period in order to check it works to specification.