As quoted in this answer:
On the fiftieth day after they began counting the Omer--that is, fifty-one days after the Exodus, all of the Children of Israel, men, women and children, over two million people, stood at Mount Sinai and received the Torah amidst great miracles and heavenly fire. They saw no form or picture of G-d, but they saw many miracles that proved that G-d is the Creator of heaven and earth. They heard G-d's voice speak and command Moses to instruct the Children of Israel on how to prepare to receive the Torah. Then they heard G-d speaking directly to them, the Children of Israel, and commanding them to keep the Torah. The Children of Israel accepted the Torah and all its Commandments, and they said: "We agree to obey, even before we hear the actual Commandments."
We are told not to believe in miracles; how do we know that the voice of God at Sinai wasn't some magician's ploy to convert the masses and how do we know that the miracles that took place at Sinai were done by God and not some magician?