Isaiah saw the LORD high and exalted, full of glory and over the whole earth. And so when Sennacherib's men stood at the gates of Jerusalem claiming that the great king of Assyria was coming and that the City was at the mercy of said king, Isaiah knew it was a lie.
Behold the truth. Study Him. Know Him intimately. And then, when the counterfeits squeak, you will know them for the liars they are.