I know that you can get your passes checked at guest relations in the parks or WDW resorts, so you could wait til you get there and get your passes checked before you enter the parks on your first visit - you can then purchase any additional days if required.
If the tickets have some sort of serial number on them you may be able to contact guest services before hand and get them checked - although I'm not sure who you would get in touch with.
Here's some useful telephone numbers:
Central Reservations Office (CRO)
for Disney Resort and Dining Reservations:00 1 407-934-7639
Walt Disney World General Information: (Includes theme park, shopping & water park hours, ticket information, special events and more): 00 1 800-492-6789 or 00 1 407-824-4321