Its easy. You ask "what is your return policy, if I find that this lens is not up to my expectations".
Other than that... what can you really do, beyond rudimentary examination?
Does it rattle? Is it clean? Is it full of dust? Are the controls smooth? Are the switches positive? Does it focus without making undue grunts and groans? Does it zoom? Check rattle at both ends and middle of zoom range.
Then go home, and check it for focus accuracy, centering, etc.
I've never gotten a "bad lens" yet from Canon, but have had one go back for focus adjustment - thats not bad, just adjustment. My 300/4 is out a little, but well within the correction range of the camera. Thats it. Lets see... 18 Canon EF lenses, one needing correction (50/1.4), and one I can now correct in the camera. I suspect I'd have not sent the 50 back to CUSA if camera correction was available at the time.
Chances are very great your lens will be just as it should be.