Plus, when this was rediscovered in 2011, it was in pretty bad shape:
The panel on which it is painted had split -- badly -- and someone, at some point, attempted to spackle it back together with stucco. The panel had also been subjected -- unsuccessfully -- to a forced flattening, and then glued to another backing. The worst offenses were crude areas of overpainting, in an attempt to hide the botched panel repair. And then there was plain old dirt and grime, centuries of the stuff. It would have taken a huge, nearly delusional leap of imagination to see a Leonardo lurking underneath the mess, yet that is exactly how the painting's story concluded.As to why people think this is a Leonardo, well, it's that old connoisseur's trick, "feel." The experts think that 1) this seems to be the original lying behind all the copies, which all vary from it in different ways, and 2) it is a lot better than any of the copies. The hand has gotten a lot of attention; Leonardo was very persnickety about the anatomy of hands, and all of the copies mess up this painting's perfection of both anatomy and perspective.
Another clue is the glass orb:
Leonardo had many imitators in 16th century Milan but their efforts are miserably below his standard. Looking at the newly cleaned Salvator Mundi, which will be exhibited at the National Gallery this autumn as a rediscovered work by Leonardo, the transparent orb is far too brilliantly painted to be the work of one of his disciples.
Leonardo was obsessed with problems like how to create translucent effects in painting, how to capture the mysteries of light – the very problems posed by painting a glass sphere.
So there you have it. The consensus of informed opinion is that this is a genuine Leonardo, but on the other hand the experts have been wrong before.