Paddy's breeder said a useful way to indicate (but not conclusive!) whether it's an itch or a habit (like biting your nails or the cuticles I guess) is to gently lift his head away while he's chewing and waggle a toy around. I've found with Paddy that if it's an itch (when he had dry skin), he's only distracted for a few seconds and then has to chew a bit more, but if it's more of an "I'm bored" habit, which has happened a few times, he doesn't return to it while you're playing with him at all - which is the signal for getting him to do some snuffling round looking for things or running round the house or garden with toys for a while.