If the attributes of God are 'omniscience’, the ability to know everything; ‘omnipotence’, the ability to do anything; and ‘omnipresence’, the ability to be present everywhere, which emphasize the infinite and transcendent nature of God, then God, in the visible sense is the Universe, Itself, and, in the invisible sense is the Mind, Itself.
When the word ‘God’ is being defined in the ways above, then how I know ‘God’ exists is because these things, which are attributed to God, obviously exist.
The fact that both the infinite, all-encompassing, Universe, and the transcendent Mind exist, proves that God, Itself, exists.
Obviously, God is not something that could be nor is above, beyond, or apart from Nature, Itself. What proves God exists, to me, are the very Things, which exist, that can be attributed to the word God.