In Tom Jones, The Foundling, we can look at fiction as history in that during the time that the story developed there was a tendency for young women who came to disgrace and became pregnant prior to marriage to leave their babies to a a) landlord b) servant c) rich family. In the case of Tom Jones, it was the latter option which made him who he was. He was a foundling which a rich family took upon themselves to raise. We know later on in the story that it was a maid and another guy who were the parents of the latter-known-as Tom Jones, yet, the way that fiction presents to us as history in the story is that it depicts a scenario much proper to a 17th to 18th century common social situation.
Fielding refers to his text as a history as he writes, although this is often to distinguish it from a romance. The depth of characterisation and in some areas, and the lively generalisation of others can present a social picture with a humorous edge. The reader can ascertain historical information from the descriptions of dress, modes of travel and accommodation and general charactersistics and beliefs of the time.
I think that fiction can be a part of the historical record in many ways. One specific way would be that when we examine fiction as the expansion of moral imagination and apply it to historical settings, our historical imagination expands as well. Fiction can be seen as history when injected into these settings. This allows the reader to have a grasp of some aspect of the historical condition through the use of fiction. In being able to identify with authors' creation of characters, context, or predicament, one has a better feel for the historical time period. One understands more of the history (non- fiction) through the fictionalized depiction of it. Fielding's work is set in the England of the Enlightenment period and, in the process of reading, one gets a better and more personalized view of the history through the fictionalized account of it.
Fiction is not history, but we can learn a lot about history by studying fiction.
If you wanted to learn about the French Revolution, A Tale of Two Cities by Dickens is a great place to start.
A good place to learn about Puritan New England would be The Scarlet Letter.
For a sense of post World War II England, try To Sir, With Love.
For a picture of the lives of European immigrants to America's Mid-West in the late 19th century, try some of the novels of Willa Cather.
And so on. But be careful. Authors of fiction are usually more interested in good plots and interesting characters than in absolute historical accuracy. Check the information you glean from the novel against the information you can find in standard historical sources.
correction: note that Tom Jones,the character is Miss Bridget's son, and is finally admitted into the Allworthys (by the end of the novel). The servant's parenthood in the plot is a scheme made up by Bridget Allworthy and her maid, Deborah Wilkins to deceive Mr Allworthy.