Actually it is very hard to tell if it faeces, bits of shell discarded during manipulation OR vomit, we discuss this at length, I suspect you have actually only seen faeces or shell fragments discarded before digestion.
Faeces are very easy to tell, regardless of the color the shape is always similar, wiry and stringy. Vomit is more diverse, little pieces of what seems to be undigested food to a white mucous-like substance. Sorry to be so disgustingly descriptive, if I happen to witness vomiting again, I will try to capture it and post the video here...