I was just listening to an audio drama called "The Surest Poison" (from the Sapphire & Steel series, most of which are quite good), and the story centers around a watchmaker in the 18th century, named Abraham-Louis Breguet, who invented various revolutionary watch mechanisms, such as the "parachute" and the "whirlwind," and who was commissioned by Marie Antoinette to make a special watch for her, including all known "complications" (anything a watch displays beyond basic date and time is known in horological circles as a "complication"). The watch was completed, but only after both the queen and the watchmaker had died, and later it was stolen in Jerusalem in the 1980s, a crime which has never been solved.
None of this detail "felt" like fiction, and some research revealed that all of it is true, including even the name of the courtier who brought the queen's commission to Breguet. Which is interesting, but it's more interesting that I could tell the difference (which also reveals that I listen to CDs without reading the liner notes, since this information is clearly laid out there).
In the Philo Vance novel The Kennel Murder Case, one clue hinges on a wounded Scottie dog, and Vance's assessment that she is no ordinary house pet but a championship-level show dog. He goes around to various dog experts and judges, and finds the clue he needs, but all of these meetings "feel" different from the rest of the novel, and I later read that the dog experts portrayed were all friends of W. H. Wright. He was a dog fancier himself, and he wrote the Philo Vance novels (under the name "S. S. Van Dyne").
I'm not sure what makes it so easy to tell the difference. I suspect (as the first paragraph up there might indicate) that the sheer amount of detail may be a factor, but it's almost certainly not the only one. Many authors fill their stories with details, and it doesn't make them seem any more real.
There is more of the school mystery posted.