How would the validity of the scripture be show?
One could ask that question about any scripture version, including the KJV.
Isaiah's a good example of that...what he said would happen came about. Isaiah 53 is an excellent chapter, for example, describing the Messiah in detail.
On the other hand, here's 53 Examples where what Joseph Smith said "didn't come to pass": AndItDidn'tCometoPass
It is easy. If the prophecies are accurate and come to pass, the scripture is valid.
Sadly there are no accurate prophecies anywhere. That tells me a lot.