It may be different for Sammael, because of the origin of his scar. That's worthy of mention in history books and essays on the Forsaken (remember, Moiraine read in TGH an early third age essay colliging information known about the Forsaken), scraps of which remain.

It all happened behind the scenes, but obviously Jordan thought up how Moiraine identified Lord Brend as Sammael, and his scar seems like the most likely way she might jump to the conclusion this was Sammael (she was already more than half-convinced that a Forsaken was in the city, she went to confirm her suspicions). Remember Jordan's Q&A. He had a terrific memory and an answer to nearly every question - it's most unlikely he didn't know how Moiraine identified Sammael, he just didn't tell, as he very often did with Moiraine. It's always been part of the way he wrote her character.



Of course Cadsuane was surprised, all Semirhage has to identify her is that she's tall for a woman and has black skin. No one but Rand could positively identify her as Semirhage.

As for Moiraine she kept most of what she knew from Rand unless nearly forced to share - until the Waste. After Rhuidean, there was Asmodean for this sort of questions (and Moiraine was spying, so she knew he was answering his questions about the other Forsaken) and Moiraine knew she was nearly out of time and focussed on giving Rand lessons on leadership and politics.

There are tons of things Moiraine never told Rand.