The idiot is me, by the way. Because I repeatedly forget in Vernon’s great trick ‘Emotional Reaction’ with the key card (see Inner Secrets of Card Magic), whether the card on the left or the card on the right is the spectator’s chosen card. I simplified the thing already by using a double-backed card as the key card (because I tend to forget the key cards as well). See the article here. Maybe you like the addition.
I marked the double-backer on both sides. In this case, I used the bone cards and shaped the bones so they point to the right (on both sides of the card). But, of course, you can think of a proper marking for any back. That way, when I fan the cards, I see the double-backer (= key card) and I see which card is the spectator’s, because the marking is pointing out to me which one of the two it is.
I know, this seems stupid, but then I am stupid sometimes, especially when doing that key card stuff. If you are not as stupid as me and can remember and spot things in performance, then you can perform without that double-backer and do it with a borrowed deck of cards …
Pass it on!