"Eight O'Clock in the Morning", a short story by Ray Nelson, first published in The Magazine of Fantasy and Science Fiction, November 1963, available at the Internet Archive. You might have read it in one of these compilations.
At the end of the show the hypnotist told his subjects, "Awake."
Something unusual happened.
One of the subjects awoke all the way. This had never happened before. His name was George Nada and he blinked out at the sea of faces in the theatre, at first unaware of anything out of the ordinary. Then he noticed, spotted here and there in the crowd, the
non-human faces, the faces of the Fascinators. They had been there all along, of course, but only George was really awake, so only George recognized them for what they were. He understood everything in a flash, including the fact that if he were to give any outward
sign, the Fascinators would instantly command him to return to his former state, and he would obey.
He left the theatre, pushing out into the neon night, carefully avoiding giving any indication that he saw the green, reptilian flesh or the multiple yellow eyes of the rulers of earth. One of them asked him, "Got a light buddy?" George gave him a light, then