Probably the best way to know if a client's happy with your work is if they keep using it.
This ad we did for my alma mater, the University of North Carolina at Chapel Hill, originally ran last March. I opened our alumni magazine and see it's back in action. That's the pay-off for doing solid work creating evergreen content.
As the granddaughter of a librarian/geneologist and the daughter of an American history major, creating an ad for the Friends of the Library that focused on the spectacular North Carolina Collection was especially important and meaningful to me.
But it's just as important and meaningful to see that the client found value in the work -- enough so that they re-ran it!