i'd say that it's as simple as this: there's someone else who isn't you with whom you share a physical body. That is broad enough that it would encompass both the disordered and healthy multiples. Of course, in some disordered systems, not everyone is AWARE that they are sharing the body with someone else, but imo, the presence of more than one distinct person sharing a physical body would seem to be the defining characteristic of multiplicity.

Re-reading your post, it looks like the question you are asking is more geared toward how you figure out for sure that there really is someone else in there when you have limited communication and/or only suspect someone else present?