In the Forest Temple, after you've killed the first 3 ghosts, the fourth (purple) ghost appears near the elevator down to the boss level. It creates 3 illusions of itself and they all circle around you. If you shoot the wrong ghost, it just disappears, but if you shoot the right one, it takes damage.
I've seen videos where people hit the correct ghost every time. How can you tell which is the correct ghost?
Best Answer
The real Meg (the final Poe Sister), will make a quick spin as soon as she splits herself into four. Her three copies won't perform the spin, so the ghost which spins is the real meg. You can see this in action here:
Meg performs her spin shortly after the 9 second mark (she's the ghost on the left). It's bit easier to see this when she spins again just after the 47 second mark.