Update 3/3/24: Reader Mark R notes that AmEx now shows clearly on the earning statement which card is getting the referrer bonus. See also this comment.
Original Post 1/9/24:
We received a neat tip from usccstrategy which simplifies figuring out which card you used when referring a friend to an Amex card:
Look at your rewards transactions showing the referrer bonus. You’ll see there a string of letters/numbers attached to each referral bonus. Example:
See in the above screenshot one of the referrals is described as “K3QE” and one is described as “K3YC”.
Next, check your referral link. Look at the full expanded version of the link. Near the end of the long link you’ll see the 4-digit code which represents the card for referrals. That code will correspond to the code you see in the login.
For example, one code I checked ends with “92-201279-K50U%3A9997&v=1&xl=cp10gx”. If that card would refer a friend, they would see “K50U” in their rewards login. Cool.