This also can be answered from the Revenge of the Sith movie.
Mas Amedda stands behind Palpatine when Yoda comes to fight him. He can clearly hear Yoda calling Palpatine "Darth Sidious", then he witnesses Palpatine when he attacks Yoda with Force lightning.
Here is the script from imsdb.com:
YODA enters, using the Force to throw two RED GUARDS against the wall,
knocking them unconscious. DARTH SIDIOUS turns his chair toward YODA.
MAS AMEDDA stands behind SIDIOUS's desk.
YODA: I hear a new apprentice, you have. Emperor, or should I call you
DARTH SlDIOUS: Master Yoda, you survived.
DARTH SlDIOUS: Your arrogance blinds you, Master Yoda. Now you will
experience the full power of the dark side.
The Dark Lord raises his arms, and LIGHTNING BOLTS shoot out,
surrounding YODA. YODA is picked up and thrown across the room,
hitting the wall and sliding down in a crumpled heap. DARTH SIDIOUS
MAS AMEDDA leaves the room. PALPATINE approaches a stunned YODA.
It's not said that Mas is already aware about Palpatine's secret identity, but regarding his lack of reaction, it looks pretty obvious to me that he knows. And if he didn't, well, he does now.