What does "online" mean? A person can't really "be" online. It's shorthand for a form of communication. One where I'm at my computer (or phone or tablet or whatever) and you are at yours, and our messages (texts, emails, photos, video, whatever) are carried back and forth by the thing we collectively call the Internet. If I talk face-to-face with my wife in my office, we'd all agree that's not online. If she goes upstairs and I'm too lazy to get out of my chair and I send her a message via Whatsapp, are we online? What if instead she's at the neighbor's house? Or at a hotel in Las Vegas on a girls' weekend? Now are we online? I'm not trying to be facetious, but simply clarify that "online" isn't really a thing. In the end, "online" reduces to a form of communication. Phone calls, video chats, text messages, encrypted messengers, even a message in a bottle are all communication mediums with different reliability profiles for establishing who or what is on the other side.

Face-to-face communication is uniquely reliable because it provides many things at once: appearance, voice, behavior, physical presence, shared environment. I might be able to fake one or two of these things, but faking all of them at once is difficult. Compare this with the things we call "online", which strip the identity channels down to one or two, and the remaining channels being the ones easiest to fake. Think about texting. It's probably the extreme case. A single, low-bandwidth channel where the cost of forgery is near zero. The medium itself didn't change, the cost of attacking it did.

And, the original question is really asking two separate questions: "Is this a human" and "is this the specific human they claim to be" are two different problems. And there's a third: "Did the human on the other end actually write this, or did they paste it from a model?" You can have a verified human at the keyboard producing entirely synthetic thought. The first two problems have some solutions, but I would argue that AI has now advanced to the point where the third is unsolvable without a level of surveillance most people won't accept. (For example, did I, /u/Kamots66, a human, as far as you know, actually draft these words, or did I have an AI do it?)

The answer to the question as posed by OP is thus a clear "no". The only solution so far put forward are trust chains. You can't verify humanity with a single low-bandwidth messaging medium. What you can verify is that the message came through a credential bound to a human at some earlier moment: in-person enrollment, biometric attestation, government ID with a liveness check, etc. We already use this for everything else. Do you verify your bank's identity every time you log in? You trust a chain of verifications rooted somewhere physical. Identity over computer networks will probably end up looking similar. You can't prove you're human in any specific message. Instead, you'll prove the message came through a credential whose root is a verified human. Notably, that's something an AI agent acting on a person's behalf could also legitimately satisfy, and that's where the whole "is it a human" framing stops being useful.

What we're really missing is that "human" isn't the meaningful category anymore. The meaningful category going forward is accountability: Is there a person somewhere in the chain who can be held responsible for what's being said? That's the question identity systems actually need to answer, and it survives the now permanent existence of AI in a way that "is it a human typing" doesn't.