As Ella's signature points out, love drives out fear. We know that God is love, and that He loves us. God drives out fear, and if it is His voice, or His desire for you, that fear will be driven out too.
The devil uses fear to try and prevent God's peace. So I find that often while following God, I will have fear hit me hard and fast and then God will go to work restoring a "peace that passes understanding".
I don't put a whole lot of value on signs, as it seems that they are often driven by our minds, not the holy spirit in us. On just a couple of occasions, God has spoken to me so clearly that I had no doubt. In the rest of the times, I would say that it is a combination of knowing God's character, and following peace.