During a battle with the Lunatic Cultist, he will summon duplicates of himself and perform a ritual. Attacking the incorrect cultist causes a Phantasm Dragon to appear. Is there a fairly easy way to determine (short of just guessing) which of the cultists is the true one? I occasionally get the feeling that the shading on the real one is slightly different, but I haven't been able to tell for certain.
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.
The real Cultist's eyes are shaped more like a "/" and the fake one's are more circular. Check the wiki for images: