Christo is God's name in Latin, will make a possessed person flinch. Despite many subsequent encounters with demons, this method of detection is not used again by the Winchesters. This has long been a bugbear of fandom.
The usual test for demons is the holy water test.
To find the disaster Demon, Dean goes around the plane saying "Christo" to various people in order to locate the demon.
Jack says "Christo" in order to test Harper Sayles.
- God in Latin is not "Christo," but "Deus." "Christos" would be "Christ" in first century Greek, translated from the Hebrew "Moshiach" (English: Messiah) both meaning the "Anointed One." It was modified and changed by the Romans into the Latin "Christus" meaning Christ, or sometimes mistakenly "Chrestus," the "Marked Man." "Christo" could arguably work to the same effect as was depicted, literally Latin meaning "by/in Christ."