I also had park hoppers from previous trips and I coulnd't remember how many days were left.
I called Disney and asked if I could read them the codes on the back and they could look it up on the comp. But no, they said that due to the large variety in the types of "pass media" the only way to have them checked is at the parks at guest services.
When I checked in 3 weeks ago for our last trip, the CM who checked us in at All Stars was able to check the number of remaining days as well as the guest services CM's at the resort.
Just remember to make a copy of the back of the passes in case they are lost/stolen (like forgotten when getting a FASTPASS).
When I checked out, I had Guest services (at the resort), verify the # of days left and used a permanent marker to write the remaining days left on them.
Hope this helps a little.
Jennifer