When viewing clients credit cards a field with the date the credit card was added would allow me to distinguish between the new credit card the client entered and one already there. There is NO way to distinguish credit cards that have been updated other than the number/expiration but some times that stays the same.
Simple option to add the date when the credit card was changed would help which was the LAST credit card your client updated without having to bother them or testing a charge and paying for the test.