Kill Prophets That Lie

And if any one again appears as a prophet, his father and mother who bore him will say to him, 'You shall not live, for you speak lies in the name of the LORD'; and his father and mother who bore him shall pierce him through when he prophesies. -Zechariah 13:3 RSV

How can one know whether or not someone is lying about 'the Lord' if everything which is known about him is by word-of-mouth? But that's obvious, this is the one biblical rule I believe was not nearly followed enough. But if all of the lying prophets were killed, the Bible would be a very thin book indeed.