Obviously only God can know the secrets of each heart. I can only make an "informed guess".
Yet Jesus gave us the method of examining the fruits to tell between true and false teachers. So, it should be a good method after all.
You seem to distrust the method given by Jesus Himself. I hope to have misinterpreted you.
15 Watch out for false prophets! They dress up like sheep, but inside they are wolves who have come to attack you. 16 You can tell what they are by what they do. No one picks grapes or figs from thornbushes. 17 A good tree produces good fruit, and a bad tree produces bad fruit. 18 A good tree cannot produce bad fruit, and a bad tree cannot produce good fruit. 19 Every tree producing bad fruit will be chopped down and burned. 20 You can tell who the false prophets are by their deeds. (Mat 7:15-20)
True and false teachers, yes. And Jesus spoke this before He died and rose again. And similar statements are made of false prophets.
What does the Bible say about those who are the true children of God?
1 John 2:21-23 KJV
21) I have not written unto you because ye know not the truth, but because ye know it, and that no lie is of the truth.
22) Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son.
23) Whosoever denieth the Son, the same hath not the Father: (but) he that acknowledgeth the Son hath the Father also.
This shows it is not the Muslim.
1 John 2:19 KJV
They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.
It's not the one who leaves the church.
Romans 6:3-7 KJV
3) Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
4) Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
5) For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
6) Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
7) For he that is dead is freed from sin.
Romans 5:1-10 KJV
1) Therefore being justified by faith, we have peace with God through our Lord Jesus Christ:
2) By whom also we have access by faith into this grace wherein we stand, and rejoice in hope of the glory of God.
3) And not only so, but we glory in tribulations also: knowing that tribulation worketh patience;
4) And patience, experience; and experience, hope:
5) And hope maketh not ashamed; because the love of God is shed abroad in our hearts by the Holy Ghost which is given unto us.
6) For when we were yet without strength, in due time Christ died for the ungodly.
7) For scarcely for a righteous man will one die: yet peradventure for a good man some would even dare to die.
8) But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
9) Much more then, being now justified by his blood, we shall be saved from wrath through him.
10) For if, when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life.
Read these passages, and then tell me that the one who denies the death of Christ, who denies the resurrection, as the Muslim does, has life in Christ.
1 Timothy 3:16 KJV
And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
Acts 20:28 KJV
Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.
Tell me that the one who denies that Jesus is God come as a man is trusting the same Jesus that we see in the Bible.
Galatians 1:3-9 KJV
3) Grace be to you and peace from God the Father, and from our Lord Jesus Christ,
4) Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father:
5) To whom be glory for ever and ever. Amen.
6) I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7) Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
8) But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
9) As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
Tell me that the one who preaches differently, that Jesus didn't die, that Jesus was a man like other men, isn't accursed. Again, be warned!
This isn't a book filled with flowery spiritual sounding speech, this is revelation from God, it's quite simply truth.
Much love!