Here is the acid test: If you are a Christian you have a spirit of wonder that permeates your life. You are always saying "how miraculous", "how interplanetary", "how unreal". You are always looking at yourself and saying, "me a Christian ... incredible, miraculous, unbelievable, a joke!!!
Perhaps this is not so much of test of who is a Christian, but who has been forgiven much. A passage from Luke in the story of the sinful woman comes to mind:
When I think of how I could have turned out (I had a sister who was an alcoholic, married seven times, and a brother who died of alcoholism as well, and another brother who drinks too much), I AM amazed that God kept me from doing a lot of things I would be ashamed of today. My life wasn’t perfect and I was no angel, but I, too, could have been a drunk or promiscuous or really screwed up. He saved me from that and I can’t thank Him enough.