More than a dozen years ago, my dad attended a business convention in San Francisco. Baseball greats Mickey Mantle and Willie Mays were hired by the sponsors to appear at the event. My dad brought my baseball, and the two retired players signed it in front of him. Do I have a home-run collectible?

— C.L. , Danbury, Conn.

I don't doubt that your dad is honest, but the collectibles marketplace won't automatically believe his story. Why? The FBI estimates that more than 80 percent of sports autographs are fakes. You need documentation, called “indisputable provenance.” What does this mean? Pull together three things. First, the name of the convention and where the ball was signed. Second, obtain a convention program and/or newspaper account saying that Mantle and Mays were there signing autographs. And third, get a notarized statement that says how you acquired the ball, and that it has always been in your possession. Even then, there will be skeptics! Value? The major price difference between the two signatures rests in the fact that Mantle is dead and Mays is not. Mantle cannot sign more material. Also, both players were retired when they signed it. Had they done so during their careers, a 10 to 20 percent premium could be added. Regionalism also impacts value — a player’s memorabilia sells higher in the market(s) in which he played. If a ball has more than one signature, its value is determined by adding a slight premium to the more valuable of the two signatures. The good news? Mantle, whose autograph is valued higher, signed the ball’s favored “sweet spot.” Estimated value: Between $750 and $850, with all documentation.