If you want to know if someone is "real" tell them an obvious lie. A human will recognize it for what it is immediately, a cursed will not. They may look real, but they are not. Only real eyes realize real lies.
"If we burn, you burn with us," I read aloud, the others gaping at the wall as they lurk behind me in uneasiness. "This must have been written here after the Infestus burned down half of New York City.