How can we know that the original Vedic scriptures have not been changed over many generations?

From: Vivek Kulkarni

Q1. Do the original texts of Vedic Scriptures (Vedas, Bhagvadgita, Bhagvatam etc) exist today? If yes, then where?

 Q2.Vedic Scriptures (Vedas, Bhagvadgita, Bhagvatam etc) were handed over from one generation to the next by aural reception. Human beings have tendency to commit mistakes as well as to cheat. What if someone might have altered / added into / deleted from contents of these texts purposefully or by mistake? How can we verify that the scriptures which we see today are exactly the same as original?