Because I am not an atheist, I hesitated answering this question. I suppose my answer is more directed to the wisdom of asking this question in the first place.
If, by asking an atheist a question about proof of God's existence, the intent is to try and somehow persuade them to believe in God with a persuasive argument, it is unlikely going to happen.
If the question is asked in order to make a believer in God seem superior to an atheist, we miss the heart of the God we claim exists.
If the question is asked to try and pull someone who is on the fence about God's existence, over to the side that believes in God...a question such as this will not likely have that result.
As a believer in God, I would hope all would find the freedom that is offered in Jesus (apart from religious strongholds both of the past and present) by being free of the brokenness that comes from living in a fallen world. It is what Jesus died for, because he loved us while we were still 'sinners'.
If an atheist holds to "being good" apart from God, we should be glad. He or she will be civil, refrain from attack and harm of others, and allow us to leave in peace as any good person would do.
In fact, I would encourage ALL of us to live the "good life"as Jesus described in Luke 6:27-36:
"But I tell you who hear me: Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you.
If someone strikes you on one cheek, turn to him the other also. If someone takes your cloak, do not stop him from taking your tunic. Give to everyone who asks you, and if anyone takes what belongs to you, do not demand it back.Do to others as you would have them do to you.
If you love those who love you, what credit is that to you? Even 'sinners' love those who love them. And if you do good to those who are good to you, what credit is that to you? Even 'sinners' do that.
And if you lend to those from whom you expect repayment, what credit is that to you? Even 'sinners' lend to 'sinners,' expecting to be repaid in full.
But love your enemies, do good to them, and lend to them without expecting to get anything back.
Then your reward will be great, and you will be sons of the Most High, because he is kind to the ungrateful and wicked. Be merciful, just as your Father is merciful.
A final thought-
1 Corinthians 2:4- My message and my preaching were not with wise and persuasive words, but with a demonstration of the Spirit's power.