Is there a simple way to identify the true Lunatic Cultist?
One way to determine which cultist is real is by playing at night. The real cultist does not emit light like the decoys, so it will be very easy to determine the correct one to attack, and only the real cultist has a health meter.
Source
The real Cultist's eyes are shaped more like a "/" and the fake one's are more circular. Check the wiki for images:
Another very easy way to identify the true Lunatic Cultist is by using a Hunter Potion, which highlights enemies in red/orange. Only the true cultist will be highlighted by the potion.