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.
Adding on to what Chippies said, having homing weapons (like the post golem possessed hatchet) can aid in finding the true cultist. This is because the homing weapon would ignore the cultist clones and only target the true one.
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.