In WH we meet Tuon's Soe'feia, or Truthspeaker, Anath. Other than having Pure Evil written all over her, what makes us think that Anath must be the long-absent Semirhage surfacing at last?
Because she says she is. After Cadsuane strips away her Illusion, Rand recognizes “Anath” and calls her out. Semi makes no move to deny it, and in fact, seems to take delight in the effect her reputation has on her captors.
“Semirhage shifted that cold smile from Rand to Cadsuane. “Why should I deny myself?” Pride dripped from every word. “I am Semirhage.” [KoD: 27, A Plain Wooden Box].
So there you go.