A cursory Google search for “cracked celluloid” seems to indicate that it is a problem, at least among fountain pen collectors. However, thinreadline is certainly correct that a hot pin should have shown a positive reaction - in the case of celluloid, that could well be in the form of bursting into flames.
All of which reinforces the difficulty of identifying ivory from a photo on a computer monitor. Sadly, my own collection has several pieces where my optimism proved to be mere wishful thinking when the item was in hand.
