The American Revolution wasn’t just fought in the boundaries of the modern United States. It was truly fought around the world. The Sons and Daughters of the American Revolution recognize that fact by allowing membership based on ancestor’s support of the American cause, no matter where they were located. That includes France.
How did France support the American cause? The country did so politically, militarily, and economically. A treaty was signed between France and the new United States in 1778. A 1936 article indicates 2112 French soldiers died in the support of the American cause. As the US State Department notes, France also kept the Continental Army supplied between 1778 and the end of the War. Ancestors who offered support through any of these efforts would qualify for the Revolutionary War lineage societies.
Where do you begin finding evidence? The Daughters of the American Revolution and the French Society of the Cincinnati both keep extensive records. So to do the publications contained in the National Library of France. It’s worth beginning with this one: https://gallica.bnf.fr/ark:/12148/bpt6k5525402h.texteImage.