[[Of course, God knows mens’ hearts - we do not.]]
We can to a pretty certain degree, tell what is in their hearts- God tells us to judge righteously to see if they be of Him or not
(Psa. 37:30) A righteous person will talk of judgment. He will not REFUSE to judge. He will talk judgment.
Corinthians 2:15 Paul says, “But he that is spiritual judgeth all things, yet he himself is judged of no man.”
1 john 4:1 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world.
“Ye shall know them by their fruits,”
John 7:24 says: “Do not judge according to appearance, but judge righteous judgment.”
john 5:30 (jesus setting an example of how to judge) “As I hear, I judge; and My judgment is righteous, because I do not seek My own will but the will of the Father who sent Me.”
While we can not see the actual heart of a person, Jesus taught us how to judge accurately, righteously, and without bias to see what is in a person’s heart, and whether they be of God or not
I John 4:1-3 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.
Heb. 5:12-14 “For though by this time you ought to be teachers, you need someone to teach you again the first principles of the oracles of God; and you have come to need milk and not solid food. For everyone who partakes only of milk is unskilled in the word of righteousness, for he is a babe. But solid food belongs to those who are of full age, that is, those who by reason of use have their senses exercised to discern both good and evil.”
and so on and so forth- We can, and indeed are commanded to judge righteously to see if the person is of God or not- it is our duty to judge their heart so as to not be taken in by a false prophet