You can use the friend code directory (accessed by clicking the search magnifying glass next to your friend code for any game) to determine who’s added you for any particular game, as well as to see new members who added their codes to the game.  There is a color coding system as well:  yellow means you’ve added someone and are waiting for them to add you, red means someone has added you and is waiting for you to add them, and green means you’ve both added each other and are ready for a matchup.  If there is no color associated with a gamer’s code, neither of you have added each other yet.