2 Timothy 3:16-17
16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
17 That the man of God may be perfect, thoroughly furnished unto all good works.
By assessing their claims against the standard of God's word. For example, Christians often say to me, "The Spirit spoke to me and said this, that and the other." When they do, I ask them where the Holy Spirit did this in Scripture. Where in the Bible does it ever say that God communicated with anyone via a private, inner voice in their mind? Nowhere. God used angels, visions, dreams, prophets, even a disembodied hand and voice, but not once does Scripture ever describe God speaking to someone as a voice in their head...
I don't believe the distinction you make holds up. While it is correct all telepathic communication to the flesh brain is demonic. That is illustrated by the following text, the devil comes and can mimic a person's "inner voice" to disguise it is him arguing against God. He can also cause images and thoughts that contradict the gospel, "taking away" the word from their hearts (2 Cor. 2:11) :
11 "Now the parable is this: The seed is the word of God.
12 "Those by the wayside are the ones who hear; then the devil comes and takes away the word out of their hearts, lest they should believe and be saved. (Lk. 8:11-12 NKJ)
Paul also speaks of this telepathic deception that affects the physical organ of the brain. "He who comes preaching another Jesus" can include "a different spirit":
3 But I fear, lest somehow, as the serpent deceived Eve by his craftiness, so your minds may be corrupted from the simplicity that is in Christ.
4 For if he who comes preaches another Jesus whom we have not preached, or if you receive a different spirit which you have not received, or a different gospel which you have not accepted-- you may well put up with it! (2 Cor. 11:3-4 NKJ)
8 But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. (Gal. 1:8 NKJ)
Telepathic messaging is how Satan blinds the mind to the gospel:
3 But even if our gospel is veiled, it is veiled to those who are perishing,
4 whose minds the god of this age has blinded, who do not believe, lest the light of the gospel of the glory of Christ, who is the image of God, should shine on them. (2 Cor. 4:3-4 NKJ)
BUT the Holy Spirit speaks to our spirit, bypassing the fleshly organ of the brain, in a "non verbal" communication. But I wouldn't rule out He might use words on occasion (Acts. 13:2-4).
Our spirits "know what God is saying" without words, just as we "know we are the children of God" without words:
The Spirit Himself bears witness with our spirit that we are children of God, (Rom. 8:16 NKJ)
Paul describes this communication as though it exists in our "conscience":
1 I tell the truth in Christ, I am not lying, my conscience also bearing me witness in the Holy Spirit,
2 that I have (Rom. 9:1-2 NKJ)
Christians commune with God the Holy Spirit all the time, often not realizing the "knowing" they should do something in Christ, is the Holy Spirit speaking to them.
13 These things we also speak, not in words which man's wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual.
14 But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.
15 But he who is spiritual judges all things, yet he himself is rightly judged by no one.
16 For "who has known the mind of the LORD that he may instruct Him?" But we have the mind of Christ. (1 Cor. 2:13-16 NKJ)
But I agree with your fundamental thought, verbal words appearing in brain are not from God.
God has nothing to do with our fallen flesh, its unclean. But our spirit is born again, a new creature in Christ---born from above and sharers of divine nature, spiritually seated with Christ in the heavenlies.
God communes with the born again spirit because it is Holy in Christ, but not with the flesh, body or brain. Any sensation, any voice experienced in unclean flesh, is not from God. In our "flesh nothing good dwells" (Rom. 7:18):
7 Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be.
8 So then, those who are in the flesh cannot please God.
9 But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His.
10 And if Christ is in you, the body is dead because of sin, but the Spirit is life because of righteousness.
11 But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ from the dead will also give life to your mortal bodies through His Spirit who dwells in you.
12 Therefore, brethren, we are debtors-- not to the flesh, to live according to the flesh.
13 For if you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live.
14 For as many as are led by the Spirit of God, these are sons of God.
15 For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, "Abba, Father."
16 The Spirit Himself bears witness with our spirit that we are children of God,
17 and if children, then heirs-- heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together.
(Rom. 8:7-17 NKJ)