Old tickets can be checked at any Disney resort Concierge desk (including at the Swan & Dolphin), at any Guest Relations location at the theme parks or DTD, or at any theme park ticket window.
Depending on how busy they are, they may limit you to checking as few as four. It is also possible that they may ask you something about the prvious usage of each ticket (the month/year of last usage, what park it was used at, etc) to verify that it is in fact your ticket before giving you the information.