I ask this because the history of places like Kashmir and Tibet have far-reaching impact on the political situation in the modern world. Depending upon the way one construes such events, biases may form and narratives may be spun, all of which will impact a nation's view of itself or it's reputation worldwide.
For example, France is often mocked by people for being conquered by Germany quite so easily in the Second World War. But there was significant opposition, and they were instrumental in helping the allies. I feel this gets glossed over in history.
Or, for another example, India and its internment of ethnic Chinese people in 1962. This barely gets mentioned, not even in Indian history text books.
It's possible, therefore, for popular history to diverge very widely from what actually happened.
I know there is resistance here to historical revisionism. Nazi sympathisers, for example, get no say here, which is clearly right. But say the issue gets slightly different -- say a historian says that China has a rightful claim to Tibet because it was part of the Qing dynasty.
How can I, as a lay person, evaluate this comment to be true or false? How can I tell a proper historian from a Jared Diamond? Are there clues I ought to look out for, certain schools for certain fields that are considered definitive sources, or something of the sort?
I trust the contributors here. They're great. But still, as a rule, how can I know something I'm learning is true and not a fiction?
how can I know something I'm learning is true and not a fiction
Most fundamentally, you can't, if all you're reading is the end product. You need to follow things back to the sources, and make your own assessments. And yes, that's time-consuming and awkward, which is why that task is outsourced to other historians - those who peer review material that is published in journals and books, and who verify that sources are what the people writing about them claim them to be - or in extreme cases, that the sources exist at all.
This is why we require people to use good sources, and provide them if asked, here on AH. If we can look at the source and go "Ok, that was published by the Journal of Verifiable History", rather than just being a random web page or Wikipedia, then we can be reasonably certain that it's reliable. There are occasional publications that are not reliable, and these are usually identified pretty quickly by other historians - people tend to read in the areas that they're interested in, and know about the areas that they're interested in. So if some less reputable journal publishes an article that mentions 14th century potato use in Ireland, I'll sit up and pay a lot of attention, look for the sources, and conclude that this publication is not reliable. And I'll communicate that to other historians, and sooner or later the publication publishes a retraction (or rather later, that journal is discredited).
However, if you're looking for an objective truth from history, you're on the wrong track. There are various areas of philosophy that will tell you that while objective truth may exist, we can't identify it. What history does is find evidence, and draw conclusions from that evidence. It is entirely possible for two historians to take the same set of evidence and arrive at different conclusions, and until a new piece of evidence arises that confirms one and contradicts the other, both are as "true" as they can be, as long as the reasoning is clear. And it's much more likely that a new piece of evidence will contradict both and someone will need to come with another new explanation.
The only definitive sources are primary ones - the objects and texts of the era you're looking at. And even those have bias, starting from as simple as "these people were literate" or "these people were rich enough to afford metal goods".
There are, to some degree, schools of historical thought, or at least identifiable threads of thought. Marxism, Great Man Theory, Cultural History, Postmodernism, and so on, are all different ways in which to look at the same evidence.
And then... with respect to claims of territory, and the like, the claim of "We owned it 500 years ago" can sometimes, sort of, be verified, but the question of whether that gives a modern state a claim to territory is not a matter for historians, but for the politicians and journalists of the current time. So no historian can say, as a historian, that China has a claim to Tibet - only that at one time, China ruled Tibet (and obviously, that not being my area of research, I'm not saying that).
In areas like Holocaust Denial, there are a number of disreputable historical practices going on. First, evidence is cherry-picked. Second, a particular bias is assumed in a great many contemporary sources. Third, bias is assigned to the modern researchers arriving at other conclusions. And any new evidence that contradicts the story being made up is ignored or denied. It is occasionally the case that one historian can identify something that has been "understood", but in view of new evidence, is no longer true, or for which a new hypothesis is better supported by more evidence. But this simply is not a thing that can happen on a large scale in widely-studied, massively-evidenced areas like WWII. If someone is trying to tell you that something completely different happened in some era, then they need to have good evidence, clear reasoning, and an explanation other than "conspiracy".
There is a kind of perception out there at the moment that scientific techniques can be used to quantify historical answers. But this simply isn't true. We have limited information about the past, and it's more limited the further back you're looking, or the less literate the people you're studying. We don't, for instance, have population information around the time of the Black Death. Censuses were not taken before or after. We have to look at things like the number of towns, the usage of them before and after, the wages paid in a few small sample areas, and then, essentially, make an educated guess at how many people lived before and after. And that is the best there can be.
As an aside, which I hope is permitted, apologies if it's not, I can answer from an archaeological perspective.
The processual vs post-processual debate in archaeology is interesting for this very reason.
Briefly explained - processualism or the 'new archaeology' focused heavily on the scientific method and sought to be objective, to produce objective facts from scientific excavation and analysis, this was around the 1960s. Post-processualism developed as a counter to this. It's influenced by post-modern thought and challenges the idea that we can be objective, it asserts that archaeology has a bias and that interpretations can be influenced by this; for example assigning a burial 'male' because it's buried with weapons without sexing the skeleton (this happened a lot in the earlier days). The post-modern/post-processual position asserts that there is no one objective truth.
The debate is still going on.
An excellent, if lengthy, book is Bruce Trigger's 'History of archaeological thought'.
From an archaeological perspective - there are many, many schools of thought, and many are heavily influenced by various philosophies. There is a general consensus on some things, I.e if you have reliable carbon dates for something. Others that you can't 'prove' in a similar black and white way, there is the 'most likely scenario based on as wide a range of evidence as we can get' but there are of course many disagreements! Much archaeology, and I assume history too, relies not only on evidence but the interpretation of that evidence - what does it mean? How does it relate to other events/bits of evidence/the general context etc? - whilst good interpretation is backed up by evidence it is still interpretation.
Separate from the "how can a layman know what's right" approach (you really can't without relying on other experts to mediate it for you — same as in physics, chemistry, etc.), the "objective truth" bit is worth a note.
There are many forms of "truth," ranging from the "patently self-evidently true" feeling you get when you see a particularly elegant demonstration of, say, the Pythagorean theorem (in which it simply must be true, no matter what you say), all the way up to the "things that may or may not be true and it's hard to say." Historians are not really in the business of the former; practically nobody is outside of mathematicians, really, though plenty of other fields try to find ways of replicating that sense of truth (to better and worse approximations — both physicists and economists have pretensions to that level of truth, but most people feel they have not done equally good jobs at it!).
Historians are somewhere in between. Our work is empirical — it is based on some form of "evidence" — but it is also interpretive. Interpretive work necessarily involves subjective judgments, questions about what feels "plausible" or "warranted" that go beyond being able to say, "I know this to be 100% true." The empiricism helps constrain it, but it does not overdetermine it: sources are tricky, there are no "raw facts" that just stand by themselves. A lot of our work involves trying to get inside the heads of the long-since dead, on the basis of a few scraps of writing that have been preserved over time — to pretend one can do such a thing wholly "objectively" is folly, at best self-deception.
A historian's truth is not the same truth as the mathematician, in other word. That doesn't mean the mathematician's truth is "better" — it is a certain type of truth and it applies to certain types of problems. Trying to apply the mathematician's standard of truth to a complex historical question ("What forces led to World War I starting?") results in mush. Different orders of questions to ask about the world, present or past, require different methodologies, and sometimes involve considerable uncertainty. History is not a "science" and does not pretend to be — it straddles social science and the humanities. While we do try to say things that are true about the world, we also recognize our own limitations, because our truths are always filtered through our subjective selves, embedded as we are within our own historical context.