But the prophet who speaks a word presumptuously in My name which I have not commanded him to speak, or which he speaks in the name of other gods, that prophet shall die. You may say in your heart, “How will we know the word which the Lord has not spoken?” When a prophet speaks in the name of the Lord, if the thing does not come about or come true, that is the thing which the Lord has not spoken. The prophet has spoken it presumptuously; you shall not be afraid of him.
— Deuteronomy 18:20-22
It does not matter if a person predicts something in the Name of the Lord because they are inspired by a demon or if they do it by the futility of their own mind. Either way, a false prophet is a false prophet. In Jeremiah’s time he was numerically overwhelmed by false prophets, but particularly irritated with Hananiah’s false predictions. God told Jeremiah:
Then the Lord said to me, “The prophets are prophesying falsehood in My name. I have neither sent them nor commanded them nor spoken to them; they are prophesying to you a false vision, divination, futility and the deception of their own minds.
— Jeremiah 14:14
These false prophets were deceived by what was in their own mind. But even after their prophecies were proven to be false, people continued to follow them.
The prophets prophesy falsely,
And the priests rule on their own authority;
And My people love it so!
But what will you do at the end of it?
— Jeremiah 5:31
When Jesus Himself warned about false prophets in the Last Days, many people think He was talking about the Jehovah Witnesses, the Mormons, the Hare Krishnas, etc. There is no doubt that the proliferation of these cults if of some prophetic significance, and they are certainly false prophets. But those are not the false prophets Jesus was warning about when He said, “If possible the elect will be deceived”. He was speaking of those that would arise to imitate Him. And the best way we can learn to spot a false prophet is to learn all we can about Jesus, who was a prophet like Moses.