How do we know that Louvre version of Mona Lisa painting is the original one painted by Leonardo Da Vinci?

Question

Please excuse me if this question seems trivial to art and painting experts but recently I became aware that besides the Louvre version of Mona Lisa, there is another version called Mona Lisa Prado. I know it's already known that Mona Lisa Prado is painted relatively at the same time when Louvre's Mona Lisa painted in Da Vinci's studio. Also, I'm aware that despite several similarities between the Louvre's Mona Lisa and the Prado version, there are significant differences between these two paintings. The most important information that I found is that based on experts' opinion, the Prado version is also painted in Leonardo Da Vinci's school probably by Salai or Francesco Melzi or other Spanish Da Vinci's pupils.

My question is that if the Prado version is painted at the same time as the Louvre's version and it comes from the Da Vinci's studio, how do we know that the Louvre's version is painted by Da Vinci himself but the Prado version is probably painted by one of his pupils? In all articles about the Prado version, the main hypothesis is that the Louvre's version is the masterpiece of Da Vinci but the Prado version is just a copy by Salai or Melzi, but nobody describes why art experts think that is the case. I really appreciate any suggestion or hint here.

Answer 1

This seems to be a general question into the nature of art appraisal: how do art experts know any work of art to be an original or a copy?

It is usually the conclusion to be drawn from the combined results of physical research (dating works, frames, researching the pigments, canvas and other materials used, checking the provenance, &c.) and stylistic research (comparison with other works, of both the presumed artist and their contemporaries, looking for signature brushwork, &c.).

After the restoration of the Prado Mona Lisa, in January 2012, a lot of new information was revealed on the matter.

Apparently, the Louvre and Prado versions of the Mona Lisa are both painted with high quality pigments, atop an underdrawing based on the same cartoon. The Prado panel has a grounding of lead white, which was rare at the time, and used by Da Vinci and his students during a specific period. The outlines of the landscape behind the sitter in both versions correspond.
These similarities imply the Prado version came from the same atelier, and either Da Vinci himself or a student made it.
Moreover, and this is especially telling, they both have very similar corrections (pentimenti) made using dark chalk, and from this the Prado experts conclude both paintings must have been made in parallel.
Most pertinent to your question, however, is the stylistic difference in the otherwise near identical composition - an obvious example being that the sfumato (the blurring of contours and gradients), typical for Da Vinci's paintings, is missing in the Prado version. From these observations it can logically be inferred that someone else must have been imitating Da Vinci's process:

"As far as we are familiar with them, Leonardo's working methods were experimental in nature whereas in the copy they are quite clear and evident. The artist is repeating what Leonardo did but in a clearer and more precise manner."

Some of the reasoning of the Prado experts is being criticized in this article on monalisa.org, which makes for a nice additional read, but the conclusions seem to be similar. The article points out how the assumed time span of the working process of the Mona Lisa must have been significantly shorter than expected (3 instead of 16 years) in order for a follower or pupil of Da Vinci to have copied it this way.