When is it appropriate to strike the pope in the forehead with a silver hammer?

Answer 1

Whether it is appropriate or not, there is an actual ritual prescribed for when the pope is discovered in a non-responsive state without the usual signs of life. A designee strikes the pope on the forehead with a ritual silver hammer (similar to the one doctors strike the knee to test for reflexes) three times while each time calling out the birth name of the pope. If the pope does not respond, he is considered to have died and the 9-day period of mourning begins.

In modern times, needless to say, a medical doctor is summoned immediately. If the doctor pronounces the pope dead, only then would rituals be done.

Answer 2

Practice abandoned:
During the twentieth century the use of the hammer in this ritual has been abandoned; under Universi Dominici Gregis, the Camerlengo must merely declare the pope's death in the presence of the Master of Papal Liturgical Celebrations, and of the Cleric Prelates, Secretary and Chancellor of the Apostolic Camera.