This post on The New Republic republished from elsewhere discusses are forgery but includes a pocket background on the history of connoisseurship:
The practice of connoisseurship originates with an Italian physician named Giovanni Morelli, who began publishing essays in German in the 1870s under a Russian name, Ivan Lemorlieff, proposing intuition-based connoisseurship.
Earlier methods had emphasized looking at iconography (study of the meaning of images). Instead, Morelli proposed a system that gave budding connoisseurs a rational, scientific way to make attributions. Focusing on overlooked details like the rendering of ears and hands, the technique placed great reliance on the expert’s capacity to read minute clues in order to extrapolate larger meanings.
His technique of observing neglected features would influence Sigmund Freud as he developed a methodology for interpreting dreams, along with Sir Arthur Conan Doyle’s Sherlock Holmes, a detective who solved crimes by looking for clues—like the unique shape of each human ear.
Update: To this Bendor Grosvenor responds on Twitter:
Oh no it doesn’t. People have been debating who painted what for centuries. Eg, Charles I asked Van Dyck re Titian. https://t.co/2rGoqlFaWz
— Dr Bendor Grosvenor (@arthistorynews) January 7, 2016
The Secret to All Great Art Forgeries (New Republic)