How can you tell if one of your friends are online on animal crossing?

go onto your friend roster. next to the person's name there should be a heart. if there is no heart there then they haven't added you yet. if there is an outline of a heart or a heatr that isn't full then that means they have added you. if the heart is full then it means you have met them (they have gone to your town or you have gone to their town). you don't know if they have defriended you (removed your friend code from their friend roster) from your friend roster, you find out if their town gate is open and you can't get in.