Oh okay - Swing Doors.
What we do here is use SetMessage();
We check if the door is locked, and then if so, display a message, otherwise, we do not.
This should work, if placed in the appropriate callback function. The one we would use is SetPlayerEntityInteractCallback, or PlayerInteractCallback as used in the Level Editor.
if(GetSwingDoorLocked(<door_name>) == true)
{
SetMessage("Sign", "CellarDoor", 0);
}
Using this, we find out if the door is locked. If so, it will display the message in CellarDoor. Just make sure to change <door_name> to the name of your door in the Level Editor, from the General tab.