So many "Mags is Luminous! Mags is Luminous!" phrases.
The real Mags goes with you to Sunscraper Spire and stays behind at the beginning of the level. She even hints that she feels like they were being watched. As soon as the Skylander reaches the first block puzzle in the second area, Luminous approaches them just after shapeshifting into Mags (as evident by the glowing on him as it disappears afterwards and then gives out hints of unusual behavior different from Mags').
After you capture Luminous, the real Mags is back at the Academy.
After you capture Luminous, the real Mags is back at the Academy.
And then there's Luminous' quote about him impersonating Mags -