either is correct. "are you a human?" is using the word human as a noun, whereas "are you human?" is using the word human as an adjective. so with the adjective you are actually referring to the nature of being a human, not actually the ghost itself. "are you human" translates more to "are you of human nature?" ah you already have that sorry lol. it depends on the context of the interaction and where you want to go with the story.

Are you a human?