Bianca Sforza by Leonardo?

This drawing was sold for about £10,000. Some 'experts' now think it is by Leonardo, and of course, the price tag is now the obligatory £100,000,000.
Alessandro Vezzosi, who wrote Leonardo da Vinci: Renaissance Man in 1996, and is the director of the Vinci Museum of Leonardo, is one of the experts who thinks this drawing could be another authentic LdV work. He could be right, but who would risk £100,000,000 in case it is not by Leonardo, and how do you prove who painted or drew something 500 years ago.