What is a writer?
I’ll reformulate the question a couple of times . Here’s one of them:
How can one tell if someone is a writer?
It’s really easy in the case of professional writers: they write; they get paid . They’ve got an output, and there’s the added ethos of gettin’ money to do something. People who’ve authored canonical works and then not much else seem to get a pass, but maybe that’s the case for all creative professions  (and for Mafiosi).
But things get sticky after that .
There are lots of reasons to write — lots of exigences, lots of audiences, lots of purposes. The idea of writing, or at least of being an author, is still magical, even now that damn near everybody  writes all the damn time . I used to teach various writing classes . In the freshman comp class I taught, the first assignment went as follows: describe a literacy event in your life. It’s a vague assignment , but that vagueness was productive. . . students could talk about writing something or reading something. Anything.
Almost all of the students talked about writing, rather than reading. (The tendency was interesting, but probably had to do with the fact that the class was a composition class, in combination with the aforementioned vagueness in the assignment ). Many students talked about creative writing that they’d done, usually for their own amusement or by way of therapy, but a lot of those student writers expressed dreams of someday being professional writers.
The idea of someone just writing things for a living appeals (among other things) to a rockstar desire in some people, I imagine. It’s nice to have people appreciate what one writes . I imagine it’d be nice to have groupies .
But the fact that lots of people want to write professionally doesn’t really answer my question about what constitutes a possibly non-professional writer.
Per my usual technique, I’d like to enter by the window rather than the door , by using a metaphor :
What is a runner?
How can you tell if someone is a runner?
This answer is easier (I hope ). There’s less mystique involved, and very few people are professional runners . As with writing, there are lots of reasons for running, but audience doesn’t enter very often into those reasons. Some people run because they’re good at it, and they want other people to see them run, but most people run for themselves, in some sense. They run to stay in shape, or for the sense of discipline, or because they like it (or those qualities I just mentioned). Or maybe for other reasons. I don’t know .
Running is less abstract than writing, so there is  less dogma involved. I asked a friend of mine another iteration of the question:
At what point would someone say “I am a runner”?
She pondered a moment, then answered “If they run two or three times a week.” That sounded pretty good, so I abandoned further research. It’s nice to get a pat answer right away : a runner is someone who runs several times a week.
So, what’s a writer?
(Before I answer, I’ve got to address the question of audience . I’ll somewhat hesitantly posit that a writer writes for an audience broader than the writer themself . I’ve recently heard several friends say something similar. Writing is or can be about communication, which seems important here, but I’d entertain opposing ideas .)
A writer is someone who writes. We’re back to writing, with its mystique and traditions and attendant dogma, so there are a variety of hardcore corollaries. A grad-school friend says something along these lines: “A writer is someone who writes every fucking day.” 
But maybe two or three times a week is enough .
1. I like repetition with variation, and the attendant nuance. And varied repetition.
2. I’m amused by the punctuation here, but I shouldn’t be.
3. Harper Lee is still described as writer, and for damn good reason. Regarding other creative professions, here’s a thought, though it’s hardly cogent: Michael Jackson was generally described as a musician. But just after Jackson died, a reporter noted that he hadn’t been famous as a musician for some years. (I can’t find the article now, dammit. I’ll append a comment if I can track it down.)
4. I mean in terms of the definition, but I imagine that things get sticky for the Mafiosi, too.
5. “Who are you calling everybody?”
6. Seriously, though, as loosely as I’m using these all-encompassing terms, the ubiquity of text messages and the internet means that a hell of a lot more people are writing now than they would have been even ten years ago. (I’m not saying that they’re all Rudyard Kipling; I’m just saying that they’re writing things.)
7. And I’d like to again, if I can find an amenable setup. Y’all send me any leads you’ve got. Don’t let that y’all put y’all off.
8. There were lots of other parameters that helped the students along, and I was there to answer questions and give guidance. More to the point, I didn’t write the assignment or design the course.
9. I just want to reiterate (for anyone who might hire me) that I didn’t design this vague assignment.
10. Hell, it’s nice when people object to what one writes, as long as they do so in a thoughtful manner that indicates that they’ve carefully read said writing.
11. I am envisioning groups of whatever sort of person a particular author finds sexually attractive, all go-go dancing around that author while he or she writes longhand at a desk in a dark study lined by hardwood shelves filled with leatherbound books.
12. A French cousin, mildly exasperated, said something similar to me when I was trying to unnecessarily circumlocute in non-fluent French. (Also, splitting infinitives can reduce adverbal ambiguity.)
13. But not that metaphor. The next one.
14. Arguing by metaphor can be a really lousy idea. But it’s fun.
15. Here’s a post on a professional runner who doesn’t finish races: http://communities.washingtontimes.com/neighborhood/rhein-time-clocking-olympian-hopefulsthe-only-plac/2012/jul/24/professional-runner-makes-living-quitter/
16. Man, I detest running.
17. I mean, y’know, maybe.
18. It’s important to befriend a variety of experts. If you can’t track down the expertise you need, just talk to someone who’s fun to drink with.
19. We can skip remuneration. We’re used to doing so, right?
20. The singular they is fine, though I confess that this iteration is grating on my nerves. I’m leaving it in anyway.
21. Perhaps by juggling, or buying them drinks.
22. There are surprisingly few hits for variations of this phrase, so I’m comfortable attributing this formulation to her. It’s in the culture, if nothing else.
23. But you have to read, too. Here’s Faulkner’s advice: “Read, read, read. Read everything — trash, classics, good and bad, and see how they do it. Just like a carpenter who works as an apprentice and studies the master. Read! You’ll absorb it. Then write. If it’s good, you’ll find out. If it’s not, throw it out of the window.”