Yesterday I asked one of our 13-year-olds (who functions at about a 5-year-old level), "Why does God love you?"
He thought for a bit and then said with a tone of wonder, "I don't know. He just does. And that makes me happy."
That attitude is a sign that you have truly come to understand God's love and working in your life.
For myself, after 32 years of learning about and revelling in my relationship with Jesus, I've come to know beyond a shadow of doubt that because of God's love for me, whatever is, is okay, i.e. whatever circumstance I face, it's going to be okay.
A sure sign that one does not know God is when h/her life is not going as s/he wants, there is not just sorrow or worry, but a deep anger and bitterness.