Hi Kim,
You ask an interesting question. How do we "test a prophet"? The Bible makes a sincere determination look easy.
Think of the prophet Jonah and the message God gave him to cry out against the city of Nineveh: "Yet forty days, and Nineveh shall be overthrown!"
Assyria was a great but evil empire—Israel's most dreaded enemy. Nineveh was the capital of Assyria. What did the people there know about authenticating genuine prophets from God? They had no prophetic checklist or divine handbook. Yet, the people of Nineveh could tell that Jonah was telling the truth. Their conviction was probably the sum of many little things. God's messenger seemed to be speaking words from God. The message was pertinent and just. Jonah could not benefit from delivering a message of disaster yet he being a foreigner in that city probably carried some risk. There may have been a mournful resignation about him that people could read. Perhaps Jonah's determination and courage was a sign. The story of Jonah makes me believe that recognizing God's prophets should be obvious.
Being able to test prophets accurately is a matter of the heart. There is a moral purpose to their messages that should be recognizable. Those who refuse to see the difference between good and evil will be blind to this. The story of King Ahab is a case in point:
For three years there was no war between Aram and Israel. But in the third year Jehoshaphat king of Judah went down to see the king of Israel. The king of Israel had said to his officials, "Don't you know that Ramoth Gilead belongs to us and yet we are doing nothing to retake it from the king of Aram?"
So he asked Jehoshaphat, "Will you go with me to fight against Ramoth Gilead?"
Jehoshaphat replied to the king of Israel, "I am as you are, my people as your people, my horses as your horses." But Jehoshaphat also said to the king of Israel, "First seek the counsel of the LORD."
So the king of Israel brought together the prophets—about four hundred men—and asked them, "Shall I go to war against Ramoth Gilead, or shall I refrain?"
"Go," they answered, "for the Lord will give it into the king's hand."
But Jehoshaphat asked, "Is there not a prophet of the LORD here whom we can inquire of?"
The king of Israel answered Jehoshaphat, "There is still one man through whom we can inquire of the LORD, but I hate him because he never prophesies anything good about me, but always bad. He is Micaiah son of Imlah." 1 Kings 22:1-8.
My favorite test in the Bible, for deciding between true and false prophets, is given in Jeremiah 23:16-18,21-22.
(For the word of God is living and active. Sharper than any double-edged sword … it judges the thoughts and attitudes of the heart. Hebrews 4:12). God's word remains a major test. Nothing has changed about false prophets since ancient times. The usual contrast between true and false prophets in Scripture is exceptionally vivid and clear:
This is what the LORD Almighty says:
"Do not listen to what the prophets are prophesying to you;
they fill you with false hopes.
They speak visions from their own minds,
not from the mouth of the LORD.
They keep saying to those who despise me,
'The LORD says: You will have peace.'
And to all who follow the stubbornness of their hearts
they say, 'No harm will come to you.'
But which of them has stood in the council of the LORD
to see or to hear his word?
Who has listened and heard his word?
I did not send these prophets,
yet they have run with their message;
I did not speak to them,
yet they have prophesied.
But if they had stood in my council,
they would have proclaimed my words to my people
and would have turned them from their evil ways
and from their evil deeds. Jeremiah 23:16-18,21-22.
Another test of a prophet is Isaiah 8:20. This familiar verse proves that agreement to prior revelation is absolutely essential.
To the law and to the testimony! If they do not speak according to this word, they have no light of dawn. Isaiah 8:20.
Ellen White believed that the decisive test of her prophetic ministry was as simple as black and white.
God is either teaching His church, reproving their wrongs and strengthening their faith, or He is not. This work is of God, or it is not. God does nothing in partnership with Satan. … There is no halfway work in the matter. The Testimonies are of the Spirit of God, or of the devil. EGW, Testimonies for the Church Volume Four, page 229.
Jesus taught that knowing the difference between true and false prophets is comparable to knowing the difference between good and bad fruit.
Watch out for false prophets. They come to you in sheep's clothing, but inwardly they are ferocious wolves. By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles? Likewise every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus, by their fruit you will recognize them. Matthew 7:15-20.
I would summarize all these Scriptures by saying that recognizing the difference between good and bad is crucial in distinguishing between true and false prophets. When the difference between good and evil is discerned, the true will be seen on the side of good, speaking on behalf of God whereas the false will found opposed to what's true and righteous and will be tearing down what God is building up.