How shall we know the word which the LORD has not spoken?

Deuteronomy 18:17-22
 
17  And the LORD said unto me, They have well spoken that which they have spoken.
18  I will raise them up a Prophet from among their brethren, like unto you, and will put my words in his mouth; and he shall speak unto them all that I shall command him.
19  And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him.
20  But the prophet, which shall presume to speak a word in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, even that prophet shall die.
21  And if thou say in your heart, How shall we know the word which the LORD has not spoken?
22  When a prophet speaks in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD has not spoken, but the prophet has spoken it presumptuously: you shall not be afraid of him.