There is a point that one can say there is a God, and that's when they know God personally in a relationship. There is proof but it's only direct proof, i.e. one person at a time and if a person seek the Creator/Truth in the first place.

Somehow you know worshipping the devil isn't good so there is something you've discerned already, even though you might not necessarily have proper proof of why.

Even people who Satan, the devil or Lucifer do good so there is something more than doing good...

Good luck on your journey :)