How do I know which card is my default payment card for Apple and Google Pay?

The first card you add to Apple Pay automatically becomes your default payment card, but you can go to Passbook anytime to pay with a different card. If you add your payment card from your iTunes account to Apple Pay, this card automatically becomes your default card in Apple Pay.

To change your default card, go to Settings on your iPhone or other Apple-Pay compatible device, then select Passbook and Apple Pay. You may then choose your default card.