There is no way to answer this question objectively. There are people who believe that they know God exists. However, there is no proof of God’s existence that is acceptable to people who do not already believe that God does exist.
There are many people who believe that they know for sure that God exists. The most extreme examples of these people are those like the boy in the article in the link below who claims that he has been to Heaven and has met Jesus Christ. Assuming that he is sincere, he knows in his own mind that God truly does exist. There are other people who are completely convinced that God has spoken to them. These people presumably know in their own minds that God exists.
The problem with this is that none of these people can offer proof that is acceptable to people who are not already believers. These people can only offer their own memories or perceptions of their experiences. They cannot offer photographic proof or anything else that would clearly establish God’s existence. People who want scientific proof of God’s existence cannot test the validity of these experiences or memories. The people who say that they have met God or had God talk to them cannot necessarily get other people to share their experiences. Thus, there is no real proof of God’s existence outside of the testimony of these people.
My view, then, is that we cannot know for sure, at least not in an objective, scientific way. There are plenty of people, however, who are convinced that they know God exists. They may well be right but they cannot prove God’s existence in any way that can stand up to scientific scrutiny.