We all know that modern internet forum users, unlike your average social media frequenter, are skeptical and they often dismiss extraordinary claims as fabrications. We all also know that these platforms - and their cousins (imageboards, IRC rooms and the like) for that matter - often have a diverse and constantly shifting userbase, which makes it pretty much impossible to know who's really who and to assess whether anyone's aforementioned fabrications are really just that.
Let's suppose that in a possibly far future, when humanity has all but abandoned their mortal bodies and uploaded their consciousness to the great net of things, criminals (actual and supposed) are punished by the government not by confining them to material boundaries but by relegating them to parts of the network where they cannot access their society anymore - that is, the past.
All offenders are sent back in time to inhabit a previous incarnation of the internet without being able to influence it beyond what is allowed to a normal user of that period of time (they're still a human mind, so no brute-forcing government sites or infiltrating nuclear missile facilities). This guarantees them a sentence that cannot end with death, as the internet has never really gone down since its modern conception and it's unlikely something like that will ever happen.
Now, here's the question: how would one of these offenders persuade modern (as in, early 21st century-based) internet users that they are in fact prisoners from the future and not just someone "making shit up"?
Keep in mind that I'm talking about Average Joe++ here, not about a scientist or someone who has enough knowledge of undeniable future events to be able to catch the attention of this period's governments. Just your average augmented consciousness who had average "common" knowledge of his time. Not unlike a modern day non-college graduate american who would certainly not be able to prove himself to be from the future if sent to the middle ages without technological devices - who the hell memorizes the answer to Millennium Prize problems or the specific impulse of observable Quasars, right?
And just giving a rundown of future technologies would probably not be enough, just think about how many speculative sites on the matter are there.
In short, what would push modern internet users, and not just a small conspiracy theory fringe group, to believe such an impossible claim? Would a list of US presidents or notable global happenings a la Nostradamus be enough for the ever skeptical internet hivemind, considering that there's no real way to confirm beyond reasonable doubt that someone who made a claim 10 years ago on the internet is the same individual who's making another claim today?
Let's assume these "criminals" manifest with an untraceable IP, or something that a first-world government itself would need to devote several million dollars to in order to crack their presumed encryption before they uncover something fishy, "I'm behind 7 proxies"-style.
I may or may not be asking this for a friend who might or might not happen to be a prisoner from the future trapped inside this site.
Edit: this is not duplicate of enter link description here, as in this question the prisoner cannot prepare beforehand or bring items with him - it's an entirely different scenario.