I have a Barcalaycard AA Red Mastercard. One benefit is a free first piece for the Primary cardholder and up to 4 accompanying companions.

I've given a secondary card to one of my offspring. He's booked to travel alone using miles from my AA account. If he shows his Red Mastercard at the check-in desk and asks for the free bag benefit, how can the check-in desk agent tell if he's Primary cardholder or Secondary?

Other than showing the credit card at the Check-in Desk what else needs to be done to get the free bag allowance?