How do you know that a book teaches truth? Because you are truth and feel it. That is what the Vedanta says. What is the proof of the Christs and Buddhas of the world? That you and I feel like them. That is how you and I understand that they were true. Our prophet-soul is the proof of their prophet-soul. Your godhead is the proof of God Himself. If you are not a prophet, there never has been anything true of God. If you are not God, there never was any God, and never will be. This, says the Vedanta, is the ideal to follow. Every one of us will have to become a prophet, and you are that already. Only know it.
The Complete Works of Swami Vivekananda/Volume 2/Practical Vedanta and other lectures/Practical Vedanta: Part 1