Differentiation is certainly difficult and there isn't a universal distinction. One might even argue the only difference is semantic, driven by a rhetorical or propagandist purpose. The distinction is also self-referential to a large degree which would mean that, like "terrorism" there can never be a universally accepted definition.

However, here's a possible simplistic distinction:

Insurgency will always contain a heavy focus on civic action by one or both of the belligerents (usually both). In a civil war civic action is minimal or absent and the focus is on military means.

Thoughts?