Let me offer two examples, one in scientific controversy, one in political.
I've recently been reading The Seven Daughters of Eve by Bryan Sykes, a book describing research, much of it by the author, that used mitochondrial DNA to investigate human prehistory, questions such as where the Pacific islanders came from and whether modern Europeans are the descendants of European hunter/gatherers who learned to farm or Middle-eastern agriculturalists who spread into Europe.
One of the incidents described in the book involved a clash between the author and a geneticist by the name of Erika Hagelberg who reported results from genetic analysis of the population of a particular Pacific island that appeared inconsistent with the accepted view of how mitochondrial DNA worked, casting serious doubt on the results of Sykes' (and other people's) research. Sykes happened to have some samples from the same island; he analyzed them and failed to find the results she had reported. He asked her for samples to analyze so that he could check her results; she did not send any. Eventually they clashed at a conference; she insisted that her results were correct but offered no explanation of why she had been unwilling to let him check them.
A year later, under pressure from her coauthors, she conceded that the results were bogus, due to a mistake in her analysis.
All of this is Sykes' account of what happened. A skeptical reader should recognize that he is getting only one side of the story and has no way of knowing how accurate it is.
Except, in this case, he does.
Out of curiosity I googled "Erika Hagelberg" and found, among other things, a book review she had written of The Seven Daughters of Eve. Its final paragraph read:
It may seem churlish to criticize a personal story of research in human evolutionary genetics designed to appeal to the public, but the tedious narrations of the lives of the clan mothers, lack of bibliography, and casual treatment of facts, rules the book out of the category of serious popular science. In the context of Sykes's commercial venture, Oxford Ancestors, which markets DNA-based genealogical information to people hungry for roots, the book makes sense as an advertising tool. However, for an accurate account of an inspiring field of science, readers should look elsewhere.
The review contained no mention of the fact that its author was herself was a character in the book and that its portrayal of her was unflattering, facts surely relevant to anyone who read the review and wanted to know whether her evaluation of the book could be trusted. That fact provides me evidence, first hand evidence, about the author of the review, evidence that supports Sykes' version of what happened.
My second example is from the world of political controversy, specifically the recent attempt by various writers to focus attention on the Koch brothers, two very wealthy men who have donated substantial amounts of money to causes the writers disapprove of. One prominent article was by Jane Mayer and appeared in the New Yorker. One of its themes was that the Koch brothers spent their money subsidizing causes that were in their corporate interest, such as opposition to government regulation of business and legislation related to climate change, and that their money was at least in part responsible for the Tea Party movement.
In this case, unlike the first, I actually knew something about the subject. The Kochs have been major funders of libertarian causes for decades. As a libertarian writer and public speaker, I have almost certainly at some point or other been paid money that ultimately came from them.
For the most part, my inside information does not tell me whether Mayer's account is true, since most of it deals with activities by the Koch brothers that I have not come in contact with. It is clear that her selection of facts leaves out things that don't fit her narrative, such as the fact that the Institute of Justice, funded in part with Koch money, has been a leading opponent of the use of eminent domain to seize property and give it to corporations, or the consistent antiwar position of the, also Koch subsidized, Cato institute, or the large contributions that the brothers are reported to have made to an ACLU attack on the Patriot Act during the Bush administration. But none of that tells the reader anything more than that the author has an axe to grind, which is in any case obvious.
There is, however, one minor detail in the article that struck me because it is demonstrably false and the author ought to have known it was false. She writes:
Indeed, the brothers have funded opposition campaigns against so many Obama Administration policies—from health-care reform to the economic-stimulus program—that, in political circles, their ideological network is known as the Kochtopus.
The term "Kochtopus" was coined by the late Samuel Konkin, a libertarian activist critical of the Kochs' influence on the movement, about thirty years before Obama was elected president. That is a fact that Mayer could have discovered with a few minutes on Google or a quick look through Radicals for Capitalism, a history of the libertarian movement that she mentions in her article. Either she did not bother to check her facts, which is a reason to be skeptical of other facts she asserts, or she deliberately transferred the term to a context that better fit the narrative she was constructing, which is a reason to distrust everything she writes.
One virtue of the Internet is that it makes it easier for me to learn, from first hand evidence, something about the honesty and competence of a source of information. A second virtue is that it makes it easier for me to convey my conclusions in a form my readers can check. I have provided links to both Erika Hagelberg's review of Bryan Sykes' book and Jane Mayer's New Yorker article, and enough information about the history of the term "Kochtopus" so that a reader can readily verify my account. And, if you follow my link to the Amazon page for Sykes' book and use their "search inside this book" option to search for the name "Erika," you can check my account of the book's description of the clash between the author and Erika Hagelberg. You thus have all the tools to do for my writing what I did for theirs.