Neither you nor I have any way of examining the full motives of another person's heart. Frankly, we are not even able to evaluate our own hearts accurately (1 Cor. 4:4). Only God can judge true repentance in the life of a human being.
Still, God gives us direction in his word to help us evaluate true repentance. The stakes of true repentance are high—literally the difference between eternal life and eternal death.
So, whether we want to gain a clear conscience for ourselves or to help shepherd someone else, we must apply God's wisdom as we seek out the genuineness of repentance.
True Repentance from Godly Grief vs. False Repentance from Worldly Grief
Here is the key text to consider:
 For godly grief produces a repentance that leads to salvation without regret, whereas worldly grief produces death.  For see what earnestness this godly grief has produced in you, but also what eagerness to clear yourselves, what indignation, what fear, what longing, what zeal, what punishment! At every point you have proved yourselves innocent in the matter.  So although I wrote to you, it was not for the sake of the one who did the wrong, nor for the sake of the one who suffered the wrong, but in order that your earnestness for us might be revealed to you in the sight of God. (2 Cor. 7:10–12)
The Apostle Paul here draws a distinction between godly grief and worldly grief. We should notice right away that we cannot judge true repentance based merely on whether someone is remorseful. The bigger question has to do with why someone is remorseful. Is this person motivated by the threats and the promises of the world, or by the threats and the promises of God?
Again, our abilities to judge between worldly grief and godly grief are limited, but God does give us some direction in his word. Here are three diagnostic tools I have developed through the course of struggling against my own sin and in shepherding people as a pastor.
True Repentance Fears God's Judgment More than Worldly Judgement
The first diagnostic tool is to evaluate what someone fears most. After the Fall, all human beings carry a complicated mixture of natural and unnatural fears, but we prioritize those fears. So, I may fear the stick of a vaccine needle to some degree, but I fear more the possibility of dying from a preventable disease. In that case, I prioritize my fear of death over my fear of a needle.
In the same way, godly grief produces true repentance because godly grief fears God’s judgment more than worldly judgment. True repentance willingly confesses sin even when confession will incur that wrath of the world. Worldly grief, on the other hand, is only concerned with the judgment of the world. Therefore, worldly grief will lie about, minimize, downplay, and ignore sin.
You may be able to hide and lie about your sin to keep yourself from trouble with your parents, your teachers, your employer, your spouse, your pastor, or your government. But, you cannot hide your sin from the God who knows everything.
Someone who is truly repentant trembles at the warning that Jesus speaks: "For what will it profit a man if he gains the whole world and forfeits his soul?" (Matt. 16:26). Acceptance in the world means nothing compared to the judgement of God, and true repentance acts accordingly.
The first diagnostic tool, then, is to ask this question: Whom do you most fear?
True Repentance Seeks to Gain God's Pleasure More than Worldly Pleasure
The second diagnostic tool is the inverse of the first. With this tool, you are trying to evaluate what someone seeks to gain the most. All of us love various things. Nevertheless at the end of the day, we prioritize our loves just like we prioritize our fears.
So, true repentance does not look like a total absence of any desire to sin. In fact, the Apostle John says that anyone who says that they no longer have sin is an unrepentant liar:
 If we say we have no sin, we deceive ourselves, and the truth is not in us.  If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. (1 John 1:8–9)
Our goal cannot be to get completely beyond sin in this life, for we will always deal with indwelling sin this side of glory. Instead, we must learn to love Jesus more than our sin. Someone who is unrepentant remains in denial about his sinfulness in order to gain or retain worldly pleasures. Someone who is truly repentant freely confesses his sins in order to gain forgiveness, cleansing from unrighteousness, and, ultimately, Jesus himself.
Here is the second diagnostic tool: What do you most want to gain?
True Repentance Seeks the Light More than Darkness
The third diagnostic tool is the application of the first and second tools. The best (albeit limited) way to evaluate whether someone is truly repentant is to ask whether that person is walking in the light.
What True Repentance Looks Like
People who are truly repentant are willing to do anything to stay in the light. True repentance does not only acknowledge the sin that others bring into the light. Instead, it goes much further, dragging out into the light what no one else knows about. True repentance eagerly exposes all kinds of secrets from the depths of their souls and skeletons from the suppressed parts of their stories.
Why do truly repentant people confess sins that no one else could ever discover? The reason is simple: truly repentant people have a keen awareness that God already knows. True repentance is the only right response to the perfect knowledge of a holy God.
Now, this doesn’t mean confessing every sin to every person. Instead, truly repentant people confess their sins to God, to people they have harmed, and to those to whom they are accountable. Living in the light requires nothing less.
What False Repentance Looks Like
Unrepentant people, however, only confess their sins when they have no other worldly option. Whether because they fear losing some worldly good or because they see an opportunity to gain some worldly good, confession is only ever a means to a worldly end.
This means that unrepentant people do not actually seek God’s light. Even when they confess to some sin they have done, they only do so to the degree necessary to accomplish their worldly goals. They are perfectly willing to keep hidden in the darkness anything that will not advance their worldly standing.
Walk in the Light
Therefore, do you walk in the light, or in the darkness? Hear the words of the Apostle John:
 This is the message we have heard from him and proclaim to you, that God is light, and in him is no darkness at all.  If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth.  But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. (1 John 1:5–7)
If we walk in darkness, we can have no part in the God who is light, in whom there is no darkness at all. But if we walk in the light, confessing our sins, we take possession of the most precious promises of the Gospel: that the blood of Jesus, God’s Son, will cleanse us from all sin.
Little children, walk in the light, as he is in the light.