don't forget that even a source which seems like it may be "facetious" or "trolling" still has plenty of value to historians in its own right - the purpose of historical sources is to give us information about history; in this case, about what life was like back then, how people lived, what they thought, etc. and a source can fulfil that role just as well if not moreso even if it isn't strictly "truthful"

i cant speak to the specific example given in that post, but an ancient joke or troll such as this, for example, could very well be based on some kind of then-popular stereotype or idea about the Irish. it does indeed seem like an "absurd claim" as you mentioned, but when the historian comes across this source, would he just throw away the source and ignore it because it's probably nonsense? definitely not - for example, something as innocuous as this could imply something about stereotypes about the Irish at the time, or possibly about certain aspects of Irish society or traditions that the joke might be based on or an exaggeration of.

a fictitious demonstrative example ive made up just now - imagine a source which documents all the jokes and rumours people told about a highly unpopular and reviled king: "king john performed oral sex on his horse!!", "king john has personally dined with the devil!!", "king john has done this or that outrageous thing!" - the value of this source would not be in telling us what king john really did or didn't do or whether the claims were truthful. rather, they would be very useful in demonstrating what people thought of king john, and if further context of the jokes was established (were the jokes told by commoners, or by court nobles, or just by everyone?), they would be even more useful in giving us information about the popularity of this fictitious example king.

this is just to point out that "trolling" or "facetious" sources can have plenty of value in their own regard and therefore don't always need to be cross-checked in the way you imagined

i welcome anyone who would like to disagree or further build on what i have said here regarding the purpose of sources when studying history

To add a bit to u/Professional-Rent-62, this might fall somewhat under the category of what Stephen Colbert calls "truthiness". If a statement of fact is striking, wonderful if it were true, the source or sources should be checked. That is often quite simple to do. For example, the old anecdote of George Washington cutting down the cherry tree was first told in Parson Weems' biography of him, and since it can't be found earlier than that, we can ascribe it to Weems. Likewise, we can trace stories/ballads about Robin Hood back to 1377, about 200 years later than King Richard I and King John. Perhaps there was an oral tradition that lasted 200 years...but maybe not.

Adding on to what's been said here about the value of sources, I would say that a lot of our primary source evaluation is contextual.

When you're evaluating any source, you want to consider context. Where and when was the source written? Who wrote the source, and who was the audience? We don't always have these answers, but knowing the circumstances surrounding the creation of a source can help us parse out the writer's perspective and consider its reliability. For example, Christian authors writing early medieval histories are going to write to a largely Christian audience with a Christian worldview, so their depictions of paganism may be informed by that.

Sometimes, you can cross-check and cobble sources together to convincingly argue that something is reliable, inasmuch as we know. Can we find other writers who give us their perspectives? Can we use archaeology to see what may have happened (for instance, supplementing written accounts of a battle with what we physically find at a battlefield)? If you have more data points, it's reasonable to be more confident in the historical contents than if you just had the one.

Another thing to keep in mind is that most, if not all source bases will have weaknesses. Historians read a ton of those, so while no one is perfect, it can help them to be mindful of those problems and to have methodological strategies to combat that. For instance, the WPA narratives as a source base are so tricky to work with. They're collected decades after emancipation by write authors, so it's possible that those memories aren't reliable, and that the subjects were not comfortable being fully honest with the writers. However, it's hard to say "throw this out as unreliable" since it's a collection of accounts by formerly enslaved people, something we don't see a ton otherwise. So, we have to supplement with other sources where we can, and otherwise we have to be discerning and try to read between the lines (or as historians call it, "against the grain") to pick out useful details and to notice the gaps. Some people get really creative with methodology by cobbling together non-traditional sources (Cheryl LaRoche uses geography, archaeology, and oral history to supplement scant resources on the Underground Railroad in her book, for example, and Saidiya Hartman leans into literary theory to fill in gaps where her sources are scant). Not everyone gets that out there with their sources, but my point here is that if a historian knows a source may not be entirely reliable, sometimes methodological strategies can offset even a source base too small and problematic too even hope to answer all of our questions 100%. I think what you'll find is that the less certain we can be about a source base, the more we have to cobble together weird sources and lean into methodology to hope to find answers to all the questions we have about the past.

I would like to add a related question: Many history books, including ones written by respectable historians, recount past events as facts, without even going into the question of how reliable their sources might be. As an example: Having recently read a lot about Stalin, I was astounded to find that many historians rely on Trotzky and other people who cant possibly be expected to be objective about Stalin. Especially the in this sub often recommended books by Montefiore on Stalin seem to me very unscientific. He continually recounts word by word conversations that allegedly took place, often between two people only.

Another example would be the much maligned roman emperors. Caligula, Nero, Commodus, unless im mistaken all of our sources are based on a few aristocratic writers who might bear a grudge simply due to an increase of taxes on the wealthy or some other reasons. So, instead of writing that these emperors were evil, wouldnt it be more sensible to write that our sources on these emperors held a grudge and we simply do not know why? Of all the history books I read, I think there were two which dealt seriously with credibility and ideology. I guess what Im asking is: Generally speaking, by what rules can historians come to any conclusions=

Speaking from a very amateur level (someone who hasn't done a PhD) I find many of the nuance in the documents I read some of the hardest things to guess at. Sarcasm, under statement, hyperbole do not travel well across written documents at the best of time, even more so if separated by a century of stylistic changes.

In my work there were many documents that I suspected contained passages of "British civil service guarded criticism" dressed as complimentary language for example. And several documents I suspected as being deliberate devil-advocate style reports. Reports that I suspected had been written in haste or annoyance portraying exaggeration and distortion. Some reports I thought were deliberate "bomb shells" writing worst case in deliberately bombastic style etc

Basically it i still find it really hard to be certain. I found reading as much as I could of documents written at the same time by the same people helped me to better understand their writing styles and what they were trying to convey. I also better understood the internal clique language used at the time. Also important to get a detailed understanding of the context of each piece of writing. Obvious things like why they were writing and for whom. Understanding this context, in my work it was that these documents were "persuasive". They were trying to get people to do things. To make decisions in their favour. To give them money etc.

Basically read more. Then come back and reconsider the document. Then read some more and do it again.