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. Being able to guess right about the identity of the one Forsaken with a very distinctive feature doesn't mean Moiraine could identify any of the others....

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 and Perrin's wolf dream). 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, and shrouding herself in mystery is part of her modus operandi. We can deduce she took big risks (she left Lan behind, and without him disobeying and following her, she may not have spotted the DH hunting her until too late) and did something which attracted his attention (because she deduced right away the DH was on her trail). It makes it sounds very likely Moiraine did something to get herself spotted by someone who reported the presence of an Aes Sedai nearby to Sammael, like getting to one of the palaces and managing to get a look from afar at Lord Brend and the Council of Nine.

It was a narrow escape, not the sort of scene where Moiraine could stop and explain in details what happened, especially that it's strongly implied the details would not show her in a good light and she would need to justify her actions (in that scene, Lan starts to remark sarcastically it's a good thing for her he disobeyed her orders and followed, and she cuts him short with a loud sniff).




Of course Cadsuane was surprised,Semirhage has no disctintive mark. That's not nearly enough, even if Cadsuane had known she was black and tall. No one but Rand could positively identify her as Semirhage, especially when none of the others had very strong motives to suspect she was a Forsaken.

As for Moiraine she kept most of what she knew from Rand unless nearly forced to share - until the Waste. She was always reserved when it concerned knowledge about the Shadow and the Forsaken. Moiraine was always afraid to lose control of the events if the others knew too much and she was always aware some knowledge might cause more problems to Rand than not knowing. There are tons of things Moiraine never told Rand - for instance most of what she knew of the KC and its interpretations. And after the Waste, Moiraine knew she was nearly out of time and focussed on giving Rand lessons on leadership and politics.