We "know" this because the "new testament" says so and "everyone knows" the "new testament" is part of the Bible because that's the Bible we inherited from our ethno-cultural group.
G-d would never allow us to inherit a false bible like everyone else has, would He?
I also hold your OT as scripture as does Christianity. Painting with a broad brush there but yes ultimately it comes to a matter of faith and knowing you Lord and God
And Pharisee-Jews "know" this as it is from their ethno-cultural group, right?
And Samaritans "know" that the Jews are wrong to add to the Torah because they inherited their Bible from their ethno-cultural group, right?
Your argument is specious at best - on what basis do Jews Add on the Prophets to the Torah? The Samaritans say that is a false bible.