The body that begins to decay and progressively is a pretty good indicator.
Well, sure. That's easy. Tougher cases are like Terri Schiavo were there was (mercifully, IMHO) actually no brain left. No possibility of having had awareness for all of those fifteen years. But there was no way to know that until after her body "died" and an autopsy could be performed. We're not good at that. The difference between a body that's just an unoccupied meat machine, and a person that's still aware and "there" is not something we're very good at defining yet.