How could you prove someone is an alien if they inhabited a human body?

^Title. I'm watching this new show, The Aliens. In it there's a simple blood test to show whether you're an alien or not. Well what would be a viable test for us, to figure out if we were human or not? If say an alien were inhabiting a human body spiritually. I'm not asking for random questions like to confuse a robot. Something legitimate. Or, a way that doesn't exist yet would interest me as well.