How do you know you're a writer?

When people are interested in what you're writing, or you write boring stuff that works. Contracts and legal documents are boring as hell, but if you are a good writer, they work, so you can be a good writer, yet not write interesting stuff. I write boring stuff and make a very good living at it. Beats teaching English to kids that hate it.

That isn't the correct question you should be asking. Anyone can write, the question is can you write well? Do you have the ability to capture humanity with the stroke of a pen (or key-bored)? Do you have the ability to put down convincing dialogue? Does your work have a point? Does it display emotion? That is the basis of a writer, it is not based upon a number of publications, nor any book list. Anyone can write, but do you have what it takes to write well?

You're a writer when you have a passion for writing. You're a professional writer when you make a living from your work (books, screenplays etc)

When you're putting stories down on paper

When you write something.

When you write stuff.

When you write...?

you just do

I don't.