Cephalopods have chitinous mandibles, otherwise known as "beaks". These tend to be the only parts of cephalopods remaining in predator stomachs: they are indigestible and the hooked shape results in them becoming lodged in the stomach lining. At least in the case of mammalian predators (seals, cetaceans), it is possible that the beaks accumulate in the stomach over a long period of time. The shape of the beaks, particularly the lower beak, is species-specific, and beak identification is a major part of diet analysis for many cetaceans: see Clarke's (1986) beak guide and many other publications. Standard measurements on the beaks can be used to estimate the original size of the cephalopod.
The upper and lower beaks of Loligo forbesi, showing starting points for standard measurements: