How did Jefferson and friends find out that Spain had ceded Louisiana back to France in 1800?

Well, there were rumors, but when Secretary of State James Madison received this letter from Rufus King, the American minister to Great Britain, on November 20th, 1801, those rumors were confirmed. It's not a long letter—more along the lines of "dropping a line"—but it imparts two crucial pieces of information.

First, it did indeed appear to be true that France was once again the rightful owner of the Louisiana Territory.

And two, even as King was scribbling down these words to Madison, Napoleon's crew was planning to head out to Saint-Domingue and proceed directly from there to New Orleans.

This was big news. Good on King for wasting no time getting the facts to the movers and shakers back in America.