I had a blogpost about the general usage of Tinder Cards on Nic Raboys blog. However, I experienced the same behaviour like you and wrote an additional article describing how to optimize the use of Tinder Cards. That includes registering for another event:
on-transition-left="transitionLeft(card)"
on-transition-right="transitionRight(card)"
On this way you should definitely get the event to which side the card was dragged/slided!