When you add somebody in your address book their name will be light grey. Once they have added you it will be black.
PS. If they have not added you the "Send Message" button will be disabled. In the Mii channel try sending a Mii, if the persons name does not come up in the list, you have not added them.
PSPS. REP REP REP, PLEASE PLEASE PLEASE!
PSPSPS. 100TH POST WOOOOOOOOOOOOOOOOOOOOO!