“You know that when you were pagans, you were led astray to the mute [incapable of articulate speech] idols, however you were led. Therefore I want you to know that no one who is speaking by the Spirit of God says, "Jesus be cursed," and no one can say, "Jesus is Lord," except by the Holy Spirit.” (1 Corinthians 12:2-3)

God does speak. He speaks in many ways. Other Scriptures tell us that He speaks through His Word, the Bible. He speaks through visions and dreams. He speaks through the creation. He speaks through a still small voice. He never contradicts Himself.

This Scripture shows us that God speaks through Gifts. The context of this verse is Spiritual Gifts that are given to members of the Body of Christ who function according to the order of the Church as defined in Scripture. (some caveats about hearing God's Voice)

We can also tell whether a person is speaking by the Holy Spirit or not.

no man speaking by the Spirit of God calleth Jesus accursed

no man can say that Jesus is the Lord, but by the Holy Ghost.

If someone curses Jesus, they are not speaking by the Holy Spirit. If someone is cursing or gossiping about or speaking evil of another member of the Body of Christ, that person is not speaking by the Holy Ghost. If someone gives glory to themselves, an organization, another person, or any thing or person other than Jesus Christ, that person is not speaking by the Holy Ghost. When someone recognizes that the ministry in another is Christ the Body, that person receives that revelation from the Holy Ghost.