Yes the passes have a serial number on the back.
But no, there is no place you can call that will look up the tickets and tell you how many days are left on them. Theoretically, they COULD, but they WILL NOT.
Disney does this on purpose, in order to kill the market for partially used tickets. They figure that only an idiot would buy tickets from a stranger if there's no way to find out whether the tickets are any good till you get to WDW.
But that also means that folks who have tickets left from previous trips have no way to check. This is also good for Disney, because it means that people are more likely to either A) Buy a bigger new ticket to make sure they have enough days when they get there, or B) Wait till they get there to buy tickets, which is more expensive than buying in advance. Either way, Disney makes a few dollars more.
The only time I have ever bought partially used tickets is from friends whom I trusted completely. I would never buy them from eBay or from a roadside stand in Orlando.