John Gill Commentary Deuteronomy 18:21

"And if thou say in thy heart, How shall we know the word which Jehovah hath not spoken?" — Deuteronomy 18:21 (ASV)

And if you say in your heart , &c] Such a thought arises in the mind, and it appears to be a difficulty, and a query is made upon it,

how shall we know the word which the Lord has not spoken ? What marks, signs, and criterions are those by which it may be known that it is not a word that comes from the Lord?