As a kid, I was very nervous to make an account on any service. I guess I could say that I was lucky to have grown up on sites like Idlechan and 4chan, because I knew what it was like not to be held down by an identity. When I was younger, I would shitpost a lot, but that didn't stop me from making posts that were more sincere in nature, or interacting with other posters with more patience than banter. If I had an account, I would argue that I could have had an established identity which would have made my posting style less schizoid, but it would ave meant that I would have to chose to either enjoy the banter of more vitriolic posting or the great sincerity that very seldom happens when swathing through the garbage.
As a kid, I remember that sometimes, when I really got btfo'd, I would sometimes even join the opposing viewpoint and argue in their favor in that very same thread. Sometimes because I believed in them and sometimes just to see if I could drum up better arguments in my favor. Either way, it helped my understand the perspective of the other posters pretty well. If I had an account, that would make me a hypocrite or someone trying to skew the discussion in their favor, but, because every post is separate, it doesn't matter who makes the argument as long as it has merit. That kind of meritocracy is helpful for being persuaded. It means that you can experiment with radical opinions without the taboo which would force you to really stick to them. It lets you flourish in a way that would never happen if you needed to save face. People with identities have egos, and their egos make them stubborn and inflexible. When there are existential repercussions to what you say, you're less experienced in debate and consequently less articulate.
And the thing is, like >>3 says, you can't necessarily tell the individual, but you can get a sense of what kind of demographic they are just by their prose. That's because you're naturally going to empathize with and emulate the mannerisms of your peers.
When I posted on the blue boards on 4chan, I could always tell when a crossposter from /b/ or whatever made a thread. I think all of us can, because they write in a way that, from our perspective, isn't succinct and relevant. You learn what's relevant and how to strip away the verbiage to your writing by participating in the community.
I mean, you can tell whether someone's posted on 4chan by simply their writing "4Chan", but you can also tell when they're doing that ironically. Same with boomer speak. You can tell when the person writing doesn't have a lot of experience communicating through text, because they write in a way where it's pretty obvious they don't have a lot of control over the words they're using. You can tell when someone is writing poorly to be facetious or ironic and when it's some out-of-touch Facebook boomer/millennial who's simply illiterate.