How do we know Désirée is not the biological daughter of the Valmonde family?

In Kate Chopin’s short story “Désirée’s Baby,” the main character, Désirée Valmonde Aubigny, is found when she is just a baby by Monsieur Valmonde on the edge of his Louisiana Plantation. The Valmondes do not have any children of their own, so Désirée is raised as their own child. Because she was abandoned, her family history is unknown, but since she has light skin, the family believes she is white. Years later, Armand Aubigny travels by the plantation and falls in love with her at first sight. The two marry, and she soon becomes pregnant. However, once she gives birth, the baby has a darker skin tone. Since her husband knows his family history—or at least believes that he does—everyone assumes that it is her family line contains African American blood. Embarrassed, Désirée takes the baby and runs away.
The plot twist comes at the end of the story. As Armand burns all trace of his wife’s existence, he finds a note written in someone else’s handwriting. In it, he learns that his own mother was actually black.

I thank the good God for having so arranged our lives that our dear Armand will never know that his mother, who adores him, belongs to the race that is cursed with the brand of slavery.