In Star Wars: Revenge of the Sith, if Mace Windu and the three other Jedi masters had waited until the Galactic Senate was in session and then came there and openly accused Chancellor Palpatine of being a Sith Lord, how would they have proven to the Galactic Senate that he was a Sith Lord?
Per the Revenge of the Sith novelisation, the fact that Palpatine is a self-professed Sith Lord (and indeed a secret Force user) isn't a crime. In fact, due to his machinations over the intervening years, changes have been made to the Constitution that guarantees the political freedom of expression even of those who profess to faiths that the Jedi find objectionable.
You're a Sith Lord!"
"Am I? Even if true, that's hardly a crime. My philosophical outlook is a personal matter. In fact—the last time I read the Constitution, anyway—we have very strict laws against this type of persecution. So I ask you again: what is my alleged crime?"
We can assume that these facts would be mentioned at his trial, but only in conjunction with proof being offered that he's been secretly controlling the Separatists, which is clearly treasonable behaviour.