You may be a novelist (and not a short story writer) if:
Your short stories regularly top out at 8000 words or more, even after you cut them for length.
Your short stories tend to have a half dozen or more named characters fully-equipped with backstories and personal agendas.
You find yourself amplifying your short stories with subplots and digressions.
You find yourself spending as much time on working out the details of your story’s background and milieu (what science fiction and fantasy writers call “doing the world-building”) as you do on working out the plot and the characters.
The milieu of your novel is bigger — more expansive, more full of implied consequences and further actions –than the story you’re telling in it.
Your trusted and reliable first reader gives you a look of deep sympathy and says, “I hate to tell you this, but what you’ve got here isn’t a short story. It’s the opening chapter of a novel.”
If this happens to you, take heart. For the natural-born novelist, novels are actually easier to write than short stories.
They just take a lot longer.