How Do You Determine Whether An Artwork Is Really A Leonardo?

To find a new Leonardo is to strike the purest vein of artworld gold. But only a few dozen of his works exist worldwide, and one of them sold recently for more than any other artwork in history. Leonardos have become a cultural currency, so to have discovered a once-lost drawing is a position of enormous potential power. If it is authentic, of course. The French seem to think so, but are they right? And how can they be sure?