If you were to ask the lying guard for ‘an example of a credit card number, expiry date and security code that weren’t the details of the other guard’, they would necessarily have to give the correct details, which the truth guard would pass on

Couldn't they just lie and give you a non-working example?

Or are you hinging your question on a variation of "don't tell me the other guard's credit card information", which you expect the lying card to contradict and tell you?