The Park hopper Tickets never expire. The days on then are good forever; and even if all the Park admission days are used, if the ticket has any "and more" days for the waterparks, those are still good after the park admissions are gone.

Any person that has access to the ticketing system (hotel check-in, concierge, guest services at all the park entrances, and inside each park, etc.) can check the number of days left on the ticket.

I've actually gone to guest services inside the park and had stacks of tickets ran and checked. I then take a sharpie and mark # of park admissions and number of water parks on the ticket.

One other thing to note: if the tickets haven't been through a gate since 2006, then they haven't been tied to a fingerprint yet (even if they were used with the hand geometry, that data was lost when Disney moved to the finger readers from the hand readers). So they are not tied to anyone right now. Once they are used at a gate, only the person who used them first can then use any remaining days past that point.

Another thing to consider (this can only be done at guest services outside the parks), is to use the current ticket to "upgrade" to a new ticket.
the way Disney deals with this is if the ticket has been used once, but still has days on it, they will credit the full CURRENT value of a like ticket minus a value for the days used. This can sometimes be used to an advantage on older tickets to up their value. (I've taken advantage of this to turn tickets into Annual passes, and have received a discount since they give current value, instead of paid value.)