How do people know God exists?

For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: ~ Romans 1:20 – KJV

Since earliest times men have seen the earth and sky and all God made, and have known of his existence and great eternal power. So they will have no excuse when they stand before God at Judgment Day.
~ Romans 1:20 – Living Bible (TLB)

There is no scientific proof of God’s existence, so the question of whether or not God exists ultimately comes down to faith. However, there are many arguments and pieces of evidence that people have put forward to support the existence of God. Some of these arguments include:

The argument from design: This argument states that the order and complexity of the universe are evidence of a designer who is God.
The cosmological argument: This argument states that the existence of the universe requires a cause and this cause is God.
The personal experience argument: This argument states that people’s personal experiences of God, such as through prayer or religious experiences, are evidence of God’s existence.

Here are some additional thoughts on the question of how we know there is a God:

We can’t see God with our eyes, but that doesn’t mean he doesn’t exist. There are many things in the world that we can’t see, but we believe they exist because we have evidence of them. For example, we can’t see air, but we know it exists because we can feel it and we can see its effects. In the same way, we can’t see God, but we can see the evidence of his existence in the world around us.
Our own experiences can be a source of evidence for God’s existence. Many people have had personal experiences that have led them to believe in God. These experiences may include feeling a sense of peace or love or feeling a connection to something greater than themselves. While these experiences can’t be scientifically proven, they can be a powerful source of evidence for those who have had them.
Ultimately, the question of whether or not God exists is a matter of faith. There is no scientific proof that can definitively prove or disprove God’s existence.

If you really want to know of the existence of God then do the below:

You will seek me and find me, when you seek me with all your heart. ~ Jeremiah 29:13

When persons seek the Lord aright, they always find him; a God hearing prayer; a God in Christ; bestowing favours upon them; granting them his presence; indulging them in communion with him; and favouring them with fresh supplies of his grace, and everything needful for them; every mercy, temporal and spiritual; that is, when they seek him in Christ, who is the only way to the Father, under the guidance and influence of the blessed Spirit; in the exercise of faith upon him and his promises; with fervency of spirit and ardour of mind; with diligence and importunity; with earnest desires and strong affections; and, as follows, with all sincerity of soul: ~John Gill