How do I know if my storyline is complete?

Apply the MECE (or NONG) test

This is a very useful test that is used in science and also applies to storylines.

The technical term is MECE, which stands for Mutually Exclusive, Collectively Exhaustive. This means that collectively the ideas in the storyline should exhaust the entire relevant universe of ideas set up by the overarching answer, and that those sub-ideas should mutually exclude each other.

Simply put, we need to check whether there are any gaps in your story overall and between your ideas, and whether any of your ideas overlap each other. The acronym for this is NONG: no overlaps, no gaps.