"Therefore being justified by faith, we have peace with God through our Lord Jesus Christ." Romans 5:1
You can tell if you have been justified by faith, you are at peace with God. What does it mean to be at peace with God? Most of all it means that in your thinking you are not seeing God as an adversary watching you closely to see if you are messing up. It means that when you think of God you think high and lofty thoughts. You think that God is working things out for you. You think that since God has gone before you, you know that even in the circumstance you are in right now (yes, I mean even the unfair situation you are in) God is in full knowledge of and God is working out the passageway through it right now. I said passageway through it because God-thinking requires you to go through it with God, not around it, over it or under it.
Being at peace with God means that when you think of God you think that God knew what He was doing when He set it all in motion and you are completely convinced that God knows what He is doing right now, in your life and those across the world. Being at peace with God is knowing in your knower that when you go into the presence of God that all that is Holy and perfect awaits you there. You understand that you go into the presence of God with expectations of being changed into a clearer image of God's Son than you were before.
You know that being at peace with God is understanding that the moments at hand are for using toward your desired end and that fighting God avails no benefit.
Being at peace with God is sublime. It is undefinable. It can only be learned from being in the presence of God. Once you recognize that God is on your side and that all of creation was designed for you and that every possible scenario was worked out for you before the earth was even made, you should find time to make it right with God and get in line with His thinking, God-thinking.