There is, of course, no way to know for certain (in a scientific sense) whether God exists. This is unknowable because God (if God exists) does not show himself if ways that can be agreed upon even by those who do not believe. If you follow the link provided below, you can see a long discussion of this question.
As the link says, the only proofs of God's existence come from "religious experience, revelation, and theological reflection." None of these is provable. One person's religious experience is another's hallucination. If a person says that God has revealed himself to that person (say, for example, Joseph Smith, the prophet of the LDS Church) people who do not share his belief will tend to say he is crazy or a liar. Theological reflection cannot be tested empirically.
So there is no way to know whether God exists. It ends up being an issue of faith -- of what a person's heart tells them.