Like practically every creative person under the sun, writers are visited by Imposter Syndrome every now and then. They stare at a blank page (or a page with a few words on it) and wonder how they ever got themselves into this situation where everyone thinks they can — and expects them to — churn out content every day or once a week or whatever. Meanwhile, trying to extract ideas from deep within their cranium is on a par with extracting teeth from a rabid Rottweiler’s backside.
That’s when the voice of doubt creeps in and starts asking “Am I really a writer?” and “Did that book / multiple books / poetry collection / essays / screenplays et al just happen by accident? Was I possessed by the ghost of Hemingway and did I accidentally fart him out?” et cetera.
Here are some signs that lets one know they are a writer, even if verbally constipated at present:
1) When you started school aged 4 or 5 (or 7 in the US) all you could bring yourself to pay attention to was stories. Nothing else mattered.
2) When it came time to do your schoolwork, if it involved retelling a story or making up your own, there was a sense of “At last, something I understand and know how to do” and you got it done in nothing flat (or it felt like no time had passed) and nothing could distract you from it, as if you were in a trance.
3) You read your eyeballs out as a kid and still do whenever there is time. (Ah, the elusive creature called Time…) You probably were at a “reading age” higher than your actual age (with a good understanding of what you read, of course).

3a) If there was no book or comic to hand, you found other reading material when doing mundane tasks: reading the cereal packet when eating, reading the toothpaste ingredients when brushing your teeth, etc.
4) You did unwise things like reading as you walked down the stairs, reading while doing the shopping with your mother and banging into people, or spending hours sitting on the toilet reading while a queue of people lined up outside crossing their legs…
5) You spent almost all your waking hours daydreaming, envisioning scenes and people in your head like a private brain-movie
6) Sometimes those daydreams would translate onto your face. Friends and relatives would wonder what you were laughing at and ask if you were feeling all right.
7) You would have entire conversations with people in your head, rehearsing what you would say if such and such a thing happened (this sign overlaps with social anxiety)
8) You would delight in strange-sounding new words, the longer and weirder the better, and collect them like stamps.
9) You were possibly introverted or shy and liked your own company (and might still do) preferring to communicate by writing rather than speaking (another thing which overlaps with social anxiety, oddly enough).
10) No matter how stuck you are, if you spend long enough not writing you will feel uneasy, perhaps agitated, as if you should be doing something and you don’t know what. Leave it for longer and your brain will give you strange, vivid dreams at night (sometimes nightmares) and wander off at tangents during the day. Leave it for long enough and it will feel as if you’ve lost a body part you didn’t know you possessed… you might find your dominant hand forming a shape like The Claw in the Jim Carrey movie Liar Liar, itching to hold a pen.