Roger Cicala of Lens rentals tests every lens after it is returned from a rental, or if a renter gets a damaged one. Unless you have a good test method for checking proper operation, have a local camera shop or canon do it for you.
The lens almost certainly sustained some internal damage, its just a matter of how much it affected the IQ and sharpness. Home grown tests for finding decentering will detect gross damage, but may not detect slight damage.
And, a cracked internal part that still works for now, will certainly fail just when you need it the most. One of the most common failures are the nylon guide bushings that guide a lens as it zooms or focuses. They can crack and keep right on working until they suddenly fall out of place and into the inner lens where it may rattle around, or jam and damage something else.
