To be absolutely sure, a signed contract is the best. Costly maybe, but then again it's worth a lot to be 100 percent sure of what you got hanging there at the wall and looking at it for years is the real thing. Even the shadow of a doubt can spoil such a thing. I always (well, almost, when the occasion is not too embarrassing) ask a celebrity for his signature when I have the chance to meet them in person.
One of my favorite autographed things is the pressbook of The Alamo which I carried to the Reunion in 1998 and had all the people sign the group picture in their respective places.
Neil Summers signed his "Candid Cowboy" books in advance, which I think is nice (did he ever do a 3rd volume?).
I would write to JW co-workers - if I had the addresses. You don't think that you could........ ;)