This is a question that could be answered many different ways depending on what period of history your studying. This is because sources for each period/region of history are different and need to be handled accordingly. I'd also like to point out that the trope "history is written by the victors" is more applicable to ancient times than modern times, since historians of modern periods have more sources available to them than accounts produced in earlier periods. Certain periods of history have had leaders who produced 'official' histories, but historians can still put on their detective hats and figure out more information than those official accounts. Simply, as others have noted, cross-checking and comparing accounts for historical review is the basis for what we, as historians do. The phrase that was hammered into me when I was an undergraduate student was "interrogate evidence" and this is at the heart of what a person should do when examining their sources. Before I explain how we interrogate evidence, I'd like to discuss how different periods of history produce different types of sources and why.

Types of Sources

The types of sources that people deal with when examining different periods of history seriously define how this question is answered. This is because different types of people in history were the only ones capable of even leaving written sources. For instance, if you were studying Britain in the year 100, Britain in the year 1200, and Britain in the year 1800, you would be examining sources with very different characteristics. Are you a professional scribe of Rome? A monk copying historical accounts? Or a businessman writing in a ledger? These examples are of a small sample of whom could have produced these items. A big reason for this was examining who could read and write. Bart Ehrman (early Christian historian) estimates that about 10% of the Roman population could read and about 3% could write around the year 0. This is an incredibly small number, meaning that only certain people could write any type of accounts. Flip to the 18th century, and literacy was incredibly high, with Kenneth Lockridge (colonial America historian) estimates that New England had nearly 100% literacy by 1800. This means that in the late 1700s, almost any could (theoretically) could leave sources. It's also worth noting that due to a myriad of reasons, sources from 1800 are much more common than sources from the year 0. This means that when doing historical investigations, there is more available information (generally speaking) to historians of later periods than historians of earlier period.

Interrogating Evidence

In this section, I'll use examples of how historians can try and interrogate evidence in order to verify or dispute history. Because my area of expertise is 18th century American Christianity, and since my own original research was used to produce my thesis that I defended this past April, I'll use my own work as an example.

My thesis entitled: The Friends They Loathed: The Persecution of Maryland Quakers During the Revolutionary War argued a few key points.[1] First, it argued that Quaker history during the Revolution almost exclusively focus on Pennsylvania and rarely focus on Quakers in other states. My second point is that Quakers in Maryland were persecuted at harsher rates than in Pennsylvania. I'm going to focus on how I was able to support this second argument in my research.

In Spring 2016, I was at the Maryland State Archives and was shown a series of records produced by Quakers during the 18th century. They key meeting minutes from a series of meetings they held called Meetings for the Suffering, which outlined detailed accounts of persecution being inflicted on them by the state government. Quakers opposed going to any war, training for militias, or taking any type of oath, and the state government had passed laws requiring all Marylanders to pay war taxes, train for militias and take loyalty oaths. This account was saying what the cost of their disobedience was.

Now, on the surface, there really aren't any 'victors' in this story. The Maryland government maybe was a victor, but they had so much internal strife that it was pretty terrible on their end. Quakers had so much persecution inflicted upon them, that they can really only be seen as being oppressed. However, most of what I was initially relying upon was Quaker records, as in, records that this small minority were producing to discuss their own plight. But, as any historian knows, you can't simply take them at their own word and leave it there. So I started searching. First, I had to substantiate their claims. I sought to see what I could compare their accounts to, which was a problem since they were only 2% of the population.

My investigation turned up a few things. First, I found accounts of 18th century Marylanders who viewed Quakers in a positive light, especially when it came to business dealings.[2] This is circumstantial and gossip, but it was a good start since it generally showed how Quakers were not known for lying or exaggerating. Next, I started tracking down the laws passed that inflicted harm on them. After that, I tried to find other accounts of what government officials said about Quakers who lived within their borders. For example, I had a letter from William Lux to Governor Thomas Johnson during the early years of the war who was complaining about how Quakers in his jurisdiction were deliberately refusing to take the loyalty oath, leading to their persecution.[3] This is a prime example of how the accountings of Quakers in 1778/1779 who were saying they were being persecuted for not taking the loyalty seemed to be reasonably true since Lux (who was in charge of Baltimore County) was citing their dissent in a letter and saying that they'd have to put pressure on them immediately. For me, looking at the 'official' accounts of the Quakers and comparing them with known laws that were passed by Maryland's government, and then compared with personal letters written between government officials, all of which substantiate the accounts of the Quakers, it made their own account much more likely to be true.

Conclusion

This is one example of how I was able to substantiate part of my thesis. If you asked 1,000 historians for examples, you'd get much more than 1,000 answers since this question is pretty open ended. However, I hope it provides some insight into how historians interrogate and check their sources.

Sources:

1] Jason Aglietti, The Friends They Loathed: The Persecution of Maryland Quakers During the Revolutionary War, (The University of Maryland Baltimore County, Published through ProQuest LLC., 2018).

2] Aubrey Land, Colonial Maryland: A History (New York: KTO Press, 1983), 98.

3] William Lux to Gov. Thomas Johnson, Dated Mar. 3 1778,” The Red Books (Annapolis: Maryland State Archives.1519. XX79), 240.

Hopefully someone will drop by with a general overview, but I didn't want you to miss this: /u/mikedash lays out his findings, then walks us through his research methods for answering the question Why did Poland have lower rates of Black Death than other European countries during the 1300s?