Here are some Bible verses that speak about that topic. There are also many other verses which talk about false prophets, but these are the main ones that address how to know if someone is true or false.
I quote the verse and then list some of the main points.
Qualifications of being a prophet.
"The LORD your God will raise up for you a Prophet like me from your midst, from your brethren. Him you shall hear, according to all you desired of the LORD your God in Horeb in the day of the assembly, saying, 'Let me not hear again the voice of the LORD my God, nor let me see this great fire anymore, lest I die.' "And the LORD said to me: 'What they have spoken is good. I will raise up for them a Prophet like you from among their brethren, and will put My words in His mouth, and He shall speak to them all that I command Him. And it shall be that whoever will not hear My words, which He speaks in My name, I will require it of him. But the prophet who presumes to speak a word in My name, which I have not commanded him to speak, or who speaks in the name of other gods, that prophet shall die.' And if you say in your heart, 'How shall 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 happen or come to pass, that is the thing which the LORD has not spoken; the prophet has spoken it presumptuously; you shall not be afraid of him. (Deuteronomy 18:15-22 NKJV)
- God speaks a message to that person who in turn tells the people.
- What the prophet says must 100% come true.
- Speaks in the name of another so-called god.
"Beware of false prophets, who come to you in sheep's clothing, but inwardly they are ravenous wolves. You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles? Even so, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Therefore by their fruits you will know them. (Matthew 7:15-20 NKJV)
- Fruits, that is, the characteristics of a non-Christian life (Galatians 5:17-21)
- No repentance
- No desire to do what is right
- No desire to please God
- No love for God ("...if you love me, you will keep (obey) my commandments..."). No fear of God
But there were also false prophets among the people, even as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Lord who bought them, and bring on themselves swift destruction. And many will follow their destructive ways, because of whom the way of truth will be blasphemed. By covetousness they will exploit you with deceptive words; for a long time their judgment has not been idle, and their destruction does not slumber. (2 Peter 2:1-3 NKJV)
- False doctrine (heresies), even heresies that hurt people and their faith
- Might even deny Jesus Christ
- Not interested in telling the truth
- Exploit, take advantage of, others
Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world. By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world. (1 John 4:1-3 NKJV)
- False doctrine of Jesus Christ
“Then if any man tells you, ‘Behold, here is the Christ,’ or, ‘There,’ don't believe it. For there will arise false christs, and false prophets, and they will show great signs and wonders, so as to lead astray, if possible, even the chosen ones. “Behold, I have told you beforehand. If therefore they tell you, ‘Behold, he is in the wilderness,’ don't go out; ‘Behold, he is in the inner rooms,’ don't believe it. Matthew 24:23-26 WEB
- May do false miracles
- Lead astray from the one, true God and His truth
Let us be aware. God bless