Q: How can we know the Word of God? 
A: We are commanded to hear, read and search the Scriptures 
(1 Pet. 2:2; Rv. 3:22; Mt. 21:42; 22:29; 2 Tim. 3:14-17)

How do we know this to be true?
Because 1 Peter 2:2 teaches this, saying:
“Like newborn infants, long for the pure spiritual milk, that by it you may grow up into salvation.”