Amos the prophet appeals not to his credentials, but to his call from the Lord as the authority for his preaching. The priest and the king, Amaziah and Jeroboam, dismiss Amos and twist his words.
But if they would hear the prophet, they would know that prophets prophesy righteousness, because prophets preach Jesus. Amos's vision of the Lord holding a plumb line paints the picture of the Lord's righteousness, which you can see fulfilled in the cross of Christ, placed squarely in the earth.