That's a very good question. The practical aspect is that there is no absolute dividing line between them. It's a language or dialect continuum, so community to community, people can readily understand one another, but as you get to the extreme ends of that, mutual intelligibility starts to break down. Some very gifted individuals with some experience of them can adapt better than others, but for many younger speakers, if they're confronted with a dialect, say Plains Cree to Moose Cree, or especially to East Cree and into Quebec, it becomes very difficult to understand one another.
On April 26th, 2018. See this statement in context.