When you know someone very well you can easily pick out their voice in a crowd. You may be on the other side of the room, but you will distinguish the words they utter very quickly. This is what happens as a result of spending time in the presence of this individual. The same is true with our Savior.
When we have spent time in the Presence of Jesus, we will easily recognize His voice, even if it is another person speaking it. The same is true for us. Others will immediately know they are hearing the heart of God speak when we open our mouths. After having spent time in His word, we will be so accustomed to His voice, we will be able to tell others what we know to be true.
Once again we can only speak out from what we know. When we know what God has spoken concerning a particular subject, then we are able to discern when it is being spoken in error, as the Holy Spirit will make it very clear. The person who believes they are speaking out truth to others may in fact be missing much of the spiritual depth. It takes knowing intimately what God’s word says to be able to share it confidently with others, and to recognize that what is being spoken is not in keeping with Scripture.
At the end of the day it is all about intimacy in relationship. Jesus made it very clear that those that know His voice will follow it. Those following Jesus will reject a voice that is not His, as they will recognize that it is not the voice that they know.
John 10:27-28 NKJV My sheep hear My voice, and I know them, and they follow Me. And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand.