This article accompanies the fable
Typically a debate over the authenticity of a Leonardo is fought over by academics. It goes like this: Carlo Pedretti (UCLA) declares it's a Leonardo and Martin Kemp (Oxford) says it isn't. The debate then drives other art critics up the wall.
In the case of this particular painting, we know that while many of Leonardo's paintings have been lost, we have two preparatory drawings that survive, as well as copies by his students. Salvator Mundi (Saviour of the World) was a familiar icon to Renaissance painters and many took a crack at it.
Below: before the restoration.