2 Peter 1:21
"For no prophecy was ever produced by the will of man,
but men spoke from God as they were carried along by the Holy Spirit."
This verse written by Peter in his letter tells us that true prophecy is made by men "carried" by the Holy Spirit. True prophecy is not what man is saying or guessing, but what God is saying. When God says it it is true.
Deuteronomy 18:18-22 tells us:
"I will raise up for them a prophet like you from among their brothers. And I will put my words in his mouth, and he shall speak to them all that I command him. And whoever will not listen to my words that he shall speak in my name, I myself will require it of him. But the prophet who presumes to speak a word in my name that I have not commanded him to speak, or who speaks in the name of other gods, that same prophet shall die. And if you say in your heart, 'How may we know the word that the Lord has not spoken?' - when a prophet speaks in the name of the Lord, if the word does not come to pass or come true, that is a word that the Lord has not spoken; the prophet has spoken presumptuously. You need not be afraid of him."
Therefore, when a prophet speaks a word that does not come true you can know that he is not being carried along by the Holy Spirit. One of the minor prophets' name is Amos, which means "burden." Obviously his parents thought he was a big burden or something to name him that. A burden, however, must be carried. Who carried him? Clearly the Lord carried him along. Can we today be carried along by the Holy Spirit? Are we not called to prophesy by Paul in 1 Corinthians 14? "Pursue love, and earnestly desire the spiritual gifts, especially that you may prophecy." Prophecy in the new testament is to build people up. Have that burden, but let the Lord carry it through you. If Paul tells us to "earnestly desire" then we can have it by faith. He can give us the grace. Ask him for it and he will carry you.