To expose fake psychics, Houdini employed a small army of undercover agents, calling them “my own secret service.” Rose Mackenberg, a private eye, was his foremost operative.
Overlooked No More: Rose Mackenberg, Houdini’s Secret ‘Ghost-Buster’ – The New York Times
You may need to register (free) to read this. A cool story about one of the people that help Harry Houdini uncovered frauds and fake psychics. She often went in disguise.