The neimodions didn't know until they were almost told by Sidious himself, to explain this better, I would say that you have to read the book, but the facial expressions are also good for this. When Sidious said: "I have the senate bogged down in procedures they will have no choice but to accept your control of the system" they realized that he would have a great power. Ending with another: "This is my apprentice; Darth Maul" I think that was the final give-away.
QGJ didn't know that it was a Sith. He said:"My experience told me it would have to be a Sith." He THINKS it's a Sith, hence: would have to be... He isn't sure but he thinks that.
I know that GL said that there was one that knew the sith were returning, but a lot of people think that one is a spy...
If I remind it correctly: there was something like that a long time ago at aldera.net (can't get there anymore 🙁 )
They gave the quote and you have to notice that GL never use the term him or something like that, it was a stron indication it was an alien.
PS; can somebody get to the site of aldera.net ????