How do I tell the difference between my own thoughts and the demons?

If you're using your mind, that's normal. I'm trying to solve an equation, I'm trying to figure something out about buying a new home: then you're using your mind. The problem is when your mind is using you. What if this happens, then I do this, then if that happens I'll do this, and then you're two hours into it and you can't sleep, or you wake up in the middle of the night. Anytime your mind is using you, you know that it's not from God.