When Count Dooku enters the room in The Revenge of the Sith, Palpatine tries to dissuade his rescuers, Obi-Wan Kenobi and Anakin Skywalker, by revealing Count Dooku's true allegiance.
Chancellor Sheev Palpatine: Get help. You're no match for him. He's a Sith Lord.
Other than to make the two Jedi doubt themselves before an important duel, why would he reveal that?
Did he think the Jedi already know Count Dooku was a Sith Lord?
The events of the previous movie, The Attack of the Clones, could lead the Jedi to think of Dooku as Sith, but there's no confirmation of that in the movie. The Jedi could still think of Dooku as a former Jedi who turned dark because he didn't like the corruption in the Senate, but going dark is not the same as becoming a Sith Lord. They could also see him as somebody who led the Separatists because he had to defeat whatever Sith Lord controlled the Senate.
Is there anything in novels or the cartoon TV series where Dooku reveals such information to the Jedi, the public, or others?
Wouldn't revealing Dooku's status also reveal that Palpatine has access to Sith knowledge?
The Sith Lords were secretive. They don't advertise their status, and would reveal themselves to somebody they trusted, or somebody they could turn. Palpatine gave a clue to his own allegiance by revealing such knowledge to Obi-Wan and Anakin. One could not imagine a Sith Lord dropping such hints carelessly.