"Are you the demon lord"
There's 4 possibilities.
1. She isn't a demon lord
2. She is a demon lord and will just lie
3. She is a demon lord and will threaten you to be quiet (or somehow make her not tell anybody)
4. She is a demon lord and doesn't care if she knows or tells anyone.
Why ask a question with a 25% chance of "yes, and I don't plan to harm you for knowing"? And it's probably even smaller than 25% chance since the demon lord is supposed to be evil.
And even with the (approx) 50% chance of just saying "no", why bother asking the question if you expect to be told "no"?