Perhaps I have been over hasty in my attack on the ideas expressed here. I want to take a second look, and see if more may be found as at least the beginning of an answer to the question of how finally one may accurately evaluate the truth of a prophet. Our question is posed clearly for us in Deuteronomy 18:20: "Any prophet (or prophets) who speaks in the name of other gods, or who presumes to speak in my name a word that I have not commanded the prophet to speak — that prophet shall die." Note the two categories of poor prophets here. The first is one who speaks in the name of "other gods." Well that appears to be simple; if some yoyo calls on some other god to support a claim, some supposed divinity who is not the one God, that person is to be disregarded. Well, duh! But not so fast. One may in fact call on some "other gods" — ideology, bigotry, arrogant certainty, etc. — to support a thought, but ferreting out the "other gods," who often come cloaked in the language of God, is far from simple.
The other category of poor prophet is the one who speaks words that God has not commanded her to speak. But that hardly solves our problem; how can I evaluate that? Now comes the answer: "When a prophet speaks in the name of YHWH, and the thing/word does not and comes not, that word is not YHWH's word; the prophet has spoken it arrogantly; do not be terrified by it" (Dt. 18:22). I have translated very literally here; the NRSV's "take place" and "comes true" are their translators' attempts to put flesh on some very general bones. Literally, one could read here in effect: "When the word spoken by the prophet is not and enters not, YHWH has nothing to do with it." In short, when a prophetic word remains empty, unrelated to the powerful love and challenge of YHWH that is shown throughout the Hebrew Bible, when it remains only self-serving and humanly undemanding and narrow, asking little if anything from its hearers, then it is plainly not YHWH's word.
Well, does that help us evaluate a prophet? We might say at least this much: words spoken in the name of God should at least pass the test of God's larger purposes for the world. God wills equality and justice for all of God's creation, and any word that flies in the face of that unique challenge surely cannot be a word from YHWH. Try that on as a test of your own next sermon, whether one you hear or one you preach. Let the word of YHWH be and enter into your words; perhaps then those words have the chance of being the word of YHWH.