The old intelligent designer idea, first expressed by Paul: "For the invisible things of him, his eternal power and divinity, are clearly understood by the creation of the world and by the things that are made so that there is no excuse".
Two: Sometimes God just makes himself known to people. I have met three people to whom this happened. One felt the power of God in Church, at an altar call. One had a bad accident and was in a coma. He had a dream where God pulled him out of a deep well into a light. At that point he actually came out of his coma. One read the Bible and suddenly everything made sense and he felt God's presence.
Three: Simply by making a decision to believe and from that point on being aware of God's presence.
I guess it is really hard to explain, but if you are willing, humble, and honest, you will come to know God exists in your own way.