[quote=Valiance]
An expert illusionist can tell by subtle clues which one is real, or at least rule many out.
[/quote]

You seem to be limiting this to your own idea of circumstances involved. I never claimed that there was an illusion that multiple beings appear, while the real one still exists among them. Limiting your mindset oh illusions to that in particular will make you weak. Plus, this situation has nothing to do with detecting whether something is or is not an illusion.