To Diamondlight:
I have often heard John 13:34, 35, (NLT) be used to describe how Christians should treat each other: "So now I am giving you a new commandment: Love each other. Just as I have loved you, you should love each other. Your love for one another will prove to the world that you are my disciples." Jesus is talking to his followers and emphasizing that they should show outstanding love to one another in the Christian congregation, and that this is a "Hallmark" of a true Christian.
But you asked the question: "In what way do we ourselves know that we are living for God? And, How can we know we are not deceiving ourselves? It seams to me, in John 13:34, 35, Jesus was mealy emphasizing the fact Christians should love each other, and under the persecution of the Romans be willing to die for one another, that is, sacrificial love that few in the western world have little concept of.
I think the true "Hallmark" of a Christian that shows you truly are living for God and are not deceiving oneself by living an easy life is clearly highlighted in Jesus' comments at Matthew 5:44-48, (NLT) "But I say, love your enemies! Pray for those who persecute you! In that way, you will be acting as true children of your Father in heaven,... If you love only those who love you, what reward is there for that? Even corrupt tax collectors do that much. If you are kind only to your friends, how are you different from anyone else? Even pagans do that. But you are to be perfect, even as your Father in heaven is perfect."
If you can show love to a stranger or an enemy, by freely forgiving someone who offends you, or cuts in front of you when driving, or god forbid steels your parking space, I am absolutely sure you are living your life for God and you are not deceived by a selfish life style.