Sonja

You dont know that?
Caspar, Melchior and Balthasar have marked the house in form of little children from the communion class dressed up as the kings going to Bethlehem and the comet itself, and if there are kids to spare, also their complete household and pack animals. They will only do it on private homes if people open the door, hear their song and give them a sum for the catholic mission into the box and optionally a little eatable treat for themselves on the way.

Generally if you see it you know the inhabitat is catholic and was home early in January of the year written on the door... thats all the math involved.