Fawn Brodie managed to find a newspaper interview with Madison Hemings, a son of Sally Hemings, who claimed that Thomas Jeffeson was his father--but said that Jefferson never treated him as his son. The 1870 census taker wrote in the margin next to Madison Hemings' name "This man is the son of Thomas Jefferson" so clearly that is what Madison told him.
Of course Madison himself did not know for sure--only knew what his mother told him.
And women never lie about paternity, right? Especially to burnish their groupie creds. LOL