I put the Channel # of the Receiver that is in the airplane on both sides of the model's Vertical Stab. All the guys in my group know what these numbers refer to and it saves them the trouble of yelling at me all the time asking "What channel you on?!" Then too if some visitor should ask what the number means, I can tell him, or her, "that is the number of that particular airplane of mine."
So if the number "49" they go away mumbling "man, he has 40 of those damned things..."