Luke looks him up and down, remaining calm outwardly but taking a moment to take the other man in. He'd been suspicious enough of coming to a party like this--he's not sure yet how meeting someone who speaks as one responsible affects that. He can't really tell anything in this crowd, and anyway, he's been unsure about how the Force works here.

He fixes the other man with his calm gaze.

"You're... not the Emperor."

It's a guess.