Short answer: God told us by various prophets, signs, wonders, miracles, and even by sending His Son to save us.
The God of the Bible declares that only He knows the future and is able to prophesy. There are over 2000 predictive prophecies fulfilled in Scripture, not just a few lucky guesses. No other so-called Scripture has any prophecy whatsoever. Jesus fulfilled 333 of them!
The God of the Bible is the only one whose Scripture has scientific, archaeological, historic, geographic, and cultural facts to back it up and invites scrutiny. Many have tried to disprove it and if you dehistoricize it, the religion would be totally discredited.
No other religious founder besides Jesus claimed to be God and proved it by miracles, signs, wonders, and teachings, especially the historic fact of the resurrection which may be the most variously proved event in all antiquity.