It is different for many atheists. I can tell you that most atheists that I have personally met or talked to grew up in a household with religion in it, and later became atheists. Although I have recently been meeting more people that grew up in an atheist or secular family. But many of us have actually deeply studied various religions, and found contradictions that steer us away from them. Meanwhile science (though imperfect) tends to better explain in a rational sense what occurs around us. I personally grew up in a Catholic household (though my parents weren't very religious, we maybe went to church 2-3 times a year). I was for a while similar to my parents being lapsed Catholics, and though I believed in God, God was never really on my mind. Eventually as I lost my faith in Catholicism, I began to have the idea that maybe God wasn't interested in us anymore. I looked at like if the earth was a flower in God's garden, would God really care if one flower wilted and died? I then from there moved to a Nietzsche-esque "God is dead" stance. I eventually moved into Deism, and then shortly after atheism. As far as how to "prove" yourself as an atheist, there would probably be several ways. I believe Daniel Dennett and Richard Dawkins set up a program called The Clergy Project for atheists that were previously minsters, priests, and clergymen. In order to become part of the program they "screen" you to make sure you are an atheist. Really the only way I could imagine they could make sure would be if they had the person denounce their faith. A truly religious person would not openly denounce their faith I would assume.