If you have Death Knighting up and use Create Shade, people can find out the real one via targetting, because it shows the glowing frames of both the Death Knighting icon -and- the Illusionary Shade.
Tested with Forced Move and Drown. Might happen with Shine Knighting too.
Steps to reproduce:
1. Use Create Shade
2. Have someone target your illusions with Forced Move or Drown, while you're in Death Knighting
3. Watch as the mystery is revealed.