It has been popular to think, believe, and teach that no one can see what's in another man's heart. We have been lead to believe that it is a sin to even try to do this, or to make pronouncements as to what we may have found. This leads to the conclusion that one's heart is sacred ground, above the eyes of other men and will eventually lead one to think that his own heart is off limits to God also. But what do the Scriptures say? Jer. 17:9 & 10, "The heart is deceitful above all things, and desperately wicked; who can know it? I, the Lord, search the heart, I test the mind..." Do you have a deceitful heart? If the answer is 'no' then you have deceived yourself. The first objection may be: "But Christ has saved me and forgiven all my sins, how can my heart still be wicked?" True, He does forgive all our sins, but no where do we find that we enter a state of sinless perfection while still living on earth. To the contrary, Romans seven gives us the reality of our life----a constant struggle with sin----while Rom. 8:1 shows us the reality of what Christ has done for us, no condemnation even for the sins we commit after we are saved (see also 1 John 1:5-10). Objection #2 may be that it is God who searches the heart and mind and not another man. Again true and no man will ever see fully into another's heart and know everything about him---that right is God's alone. But there's more.
Then we have the words of the Lord Jesus in Matt. 12:33 & 34; "...For out of the abundance of the heart the mouth speaks" Uh oh! Someone left the door open. Luke 6:45; "A good man out of the good treasure of his heart brings forth good; an evil man out of the evil treasure of his heart brings forth evil. For out of the abundance of the heart his mouth speaks." The mouth will speak whatever dwells in the heart, good or evil. Matt. 6:15-20 concludes this teaching with: "Therefore by their fruits you will know them." The mouth of a man is an open door to his heart for all to see inside, so open is it that the door itself may as well be removed leaving only the gaping hole that the door once filled. James teaches that "no man can tame the tongue" and goes further to say that "with it we bless our God and Father, and with it we curse men who have been made in the similitude of God." James 3:8 & 9. So it is not just the lost that have this tongue problem, but the saved as well. A man only deceives himself to think otherwise. Not one of us (the redeemed) can escape the reality of an untamable tongue, but thank God that "He giveth more grace!" John 4: 13 & 14 says that a saved man has "a fountain of water springing up into everlasting life." And John 7:38 says: "He who believes in Me, as the Scripture has said, out of his HEART will flow rivers of living water." Thank God the heart is changed, not simply repaired, but made completely new, a "new creation" (2 Cor. 5:17). So why do we still have problems with sin? A loaded question to which there is no real and satisfactory answer for now, perhaps we shall know more fully when we finally sit at our Lord's feet around His throne. Part of the "answer" may lie in the fact that God loves for His people to seek Him, daily, constantly 24/7 (pray without ceasing). The Lord said of David that he was a "man after His own heart" (Acts 13:22), perhaps one reason for this was that David made a habit of calling out to His God on a regular basis and often would say "Search me O God and know my heart, try me and know my thoughts; and see if there is any wicked way in me, and lead me in the way everlasting" Psa. 139: 23 & 24. And "Let the words of my mouth and the meditation of my heart be acceptable in your sight, O Lord, my strength and my redeemer" Psa. 19:14. Nor was David a stranger to repentance, a broken and contrite heart before His God, Psa. 51:1-19.
It's always a good day for a believer to examine his own heart, not to "see whether you are in the faith", but whether there may be a hidden idol that demands attention, or a sin that plagues. Lay your heart bare before Him and ask Him to examine your heart. You will find nothing but kindness and longsuffering extended to you along with a renewed humility and gratitude for the Lord's Mercy and Love.