Provenance is Proof
No matter how authentic a purported Picasso or Rembrandt looks, you wouldn't plunk down millions of dollars for a painting without seeing paperwork that details all the hands that the artwork has passed through over the years, and proves it isn't a forgery. Similarly, when you're presented with a purported historical fact, you can track it back though the past and make sure that the story hasn't been changed, or if you're interested enough, even figure out how it's evolved over time -- who tinkered with the truth and why.
There was a time when such a quest would have meant perusing dusty volumes on the back shelves of libraries or squinting at microfilm. But today, thanks to the Internet and the rise of online databases and search engines, you can do a pretty good job of it with a few keystrokes. From 2008 to 2011, Google compiled a searchable database of old newspapers from around the world dating back into the 1800s, and though that project is no longer adding content, nearly a million pages from 2,000 papers remain available for searching [source: Keller]. Additionally, there are a number of other databases available if you don't mind paying for them, such as the New York Times' online database of articles dating back to the mid-1800s. Another such online source is Newspaper Archive which includes 120 million articles from newspapers from all over the U.S. and some foreign countries, dating back to the 1600s [source: Newspaper Archive].