Short story about a journalist and retired rich actress who disclose fake hollywood psychic

Sounds like "Inside Job" by Connie Willis.

A debunker and journalist and his female accomplice try everything they can to disprove and discredit a psychic. The psychic is written out to be an obvious show-business fraud, in it for the money and the stardom.

The debunkers have battles with the psychic at her extravagant shows, without success. (Probably because it is more difficult to definitively disprove a concept.)

One night the fraudulent psychic appears to channel H. L. Mencken. Mencken was a notorious, hard-headed skeptic who would have hated everything about the phony psychic. "Mencken", if such it actually was, blasted everything about her -- and her entire loyal audience heard.

The story leaves it ambiguous whether it was real channeling or not. But that type of "psychic" would never have willingly done a phony channeling job of someone who so strongly showed her up for the phony she was. It is almost like a karmic "she was right for one time in her life."

I can not find it online to quote it the way people on this board like, as if people want to read the story here rather than in its native land. But this should get you going.

Connie Willis, is, of course, her usual brilliant self.