Count Dooku har känt om Sidious / Papaltine så länge han var hans lärling.
Jag har inte läst de två böckerna som dessa händelser spänner över, så jag går utifrån citaten, men enligt Wookieepedia ...
Under Darth Plagueis - som uppstår mellan episoder 1 & 2:
Palpatine did not reveal himself at that point, unsure of Dooku's true allegiances. However, with the death of his Master Plagueis at his own hand, and the presumed death of Darth Maul at the hands of Obi-Wan Kenobi, Palpatine needed an ally to further his own plans for galactic domination.
During Nute Gunray's final trial out of four tribunals for his role in the Naboo Crisis, Dooku learned from Gunray about Darth Sidious' role behind the scenes in the crisis, to which Dooku then promised to supply this information to the Jedi Order. It wouldn't be until 22 BBY that he would eventually reveal this information to the Jedi, and even then, only to Obi-Wan Kenobi, who would later supply this information to the Jedi Order.
However Dooku's fate as a Sith apprentice was sealed before it had begun, as Sidious had already met the young boy Anakin Skywalker, the supposed Chosen One of Jedi prophecy. Realizing Skywalker's one-of-a-kind potential in the Force, Sidious had already made plans for Skywalker to serve as his final apprentice. As such, Sidious decided to keep Dooku only as a place holder until Skywalker had fully matured, while still using the fallen Jedi's unique talents to further the Sith agenda.
På grundval av dessa punkter kan det hävdas huruvida Papaltine faktiskt har avslöjat sin dolda identitet.
Men nästan omedelbart efter, under Labyrint of the Evil träffas de igen - den här gången med Papaltine / Sidious förklarar hans fulla planer (minus delen om Dooku så småningom ersätts av Skywalker).
Shortly after their meeting on Coruscant, Dooku was approached directly by Darth Sidious. They had a long discussion, and Dooku found that Palpatine's goals were not dissimilar to his own. Despite the recent death of Dooku's apprentice Qui-Gon Jinn at the hands of Darth Maul, Darth Sidious found it surprisingly easy to convince Dooku to ally with the Sith. After seventy years in the Jedi Order, Dooku abandoned his friends and the Jedi way for the dark side of the Force.
Under the tutelage of Sidious, Dooku was renamed Darth Tyranus and became a Dark Lord of the Sith.