We tell someone's character by their attitudes, thoughts, and actions. We can assess the actions of malicious/evil spirits by the harm they do. We can assess the character of God by the thoughts, motives, and actions that are told to us in the Bible and that we see in history. We can use our brains to their fullest capacity.
It would be a contradiction in terms and ontology if God could lie or were evil, and therefore it's impossible.
1. God is ideally definable as the ideal being, greater than any other imaginable being, and perfect in his attributes. Therefore if God could lie, he could not be God, and so a self-defeating and self-contradictory proposition.
2. Without truth, there is no morality. Without morality, there is no goodness or right/wrong. Without the possibilities of truth, goodness, or right, there can be no God. It goes against every reasonable and acceptable definition of God.
3. The Liar's Paradox (
https://en.wikipedia.org/wiki/Liar_paradox) shows that if God is a liar, he is a living ontological contradiction, which is logically impossible. A cannot equal non-A.