On the header section of your My Subscriptions cards, you can see which card will be charged.
Navigate to your My Subscriptions page.
Look for the subscription card where you are wondering which payment method applies.
At the top of the card, you will be able to see the date of your upcoming charge.
Look for the Payment Method field. The card listed here will be the card which will be charged for your upcoming box.
If you are looking for a previous order, navigate to the Subscription History on the subscription card, and look for the Payment Method field. The payment method listed would have been charged for your previous order.
To change your payment method, visit this article for information on how to do so.