His physical mediumship was investigated by notable psychical researchers such as Harry Price, Albert von Schrenck-Notzing and Eric Dingwall, the Research Officer of the Society for Psychical Research. Willi, like his brother Rudi, was also caught cheating in the séance room. Warren Vinton, who investigated Rudi, also attended sittings with Willi in 1926. According to Vinton there was movement of some curtains and a sheet of paper which were performed by Willi's blowing.
Author Roy Stemman documented a picture of Willi in a séance with a "faked cloth phantom".
- Thruelsen, Richard. (1961). Adventures of the Mind. Knopf. p. 325. "An analogous case was that of the Schneider brothers, Willy and Rudy, who performed a series of teleplastic manifestations that were carefully examined by Professor Przibram of the University of Vienna and discovered to be nothing but deceptions."
- Rawcliffe, Donovan. (1988). Occult and Supernatural Phenomena. Dover Publications. p. 313. ISBN 978-0486255514 "Rudi, and his brother Willi, had been repeatedly and comprehensively exposed as fraudulent tricksters, yet such is the faith of those imbued with a penchant for the mysterious that both spiritualists and psychical researchers alike continued to believe in the possibility of their supernatural powers. "Ectoplasm" and other "teleplastic manifestations" were their main specialities. These, however, were investigated by Professor Przibram of the University of Vienna and found to be nothing more than deceptions."
- Vinton, Warren Jay. (1927). The Famous Schneider Mediumship: A Critical Study of Alleged Supernormal Events. In C. K. Ogden. (1995). Psyche: An Annual General and Linguistic Psychology 1920-1952. Routledge/Thoemmes Press. ISBN 978-0415127790
- Stemman, Roy. (1978). The Supernatural. Danbury Press. p. 133
- Trevor H. Hall (Oct 1978). Search for Harry Price. Gerald Duckworth and Company. pp. 136–140. ISBN 0-7156-1143-7.
- Peter Underwood (1978). Dictionary of the Supernatural (hardback ed.). ISBN 0-245-52784-2.