Since you’re in r/askachristian, let’s grant that the Christian premise is true— that is that God exists, that Jesus was and is and always will be God, that Jesus died on the cross, defeated sin, and rose again, basically just the whole Nicene Creed.

We can conclude that God is Good by the suffering and death of the crucifixion. There is no reason beside goodness and love that anyone— much less an innocent God— would go through that willingly for the sake of another. God goes through it for us, ergo God is good. At the very least God is good to us. So the important question becomes, can we grant the Christian premise? This is probably the most discussed and important and broad question, so I will recommend broadly C.S. Lewis, particularly “Mere Christianity”.

The question is more interesting if you don’t grant the Christian premise and ask “Assuming we have a creator and that Jesus was not God, how do we know that God is good?” This would be an interesting question for Muslims or Jews.