There was a argument in the early days of Psychical Research that mediums were using telepathy from unconscious minds who were at the sittings.
The Reverend Charles Drayton Thomas, a Methodist minister who became a psychic researcher, spent many years as a proxy sitter investigating the mediumship of Mrs Leonard and recording his results for the Society for Psychical Research. He would go to a sitting knowing only the name of the deceased and the name of the person who desired communication.
In one instance in 1936-37 Thomas went to four sittings on behalf of Emma Lewis, a person he did not know. Through the medium, Mrs Leonord, he was able to gain seventy pieces of information which Emma later felt confirmed beyond all doubt that it was her father, Frederick William Macaulay, who was communicating.
The only possible objection a skeptic can make in relation to proxy sittings is fraud. There is no other possible or probable explanation for the information coming through the medium about someone who has passed on and who had no connection whatsoever with any of the sitters who were with the medium at the time.
But so far in the last fifty years or so, no one has been able to even suggest fraud in the proxy sittings conducted by the Reverend Drayton Thomas. Again, psychic researchers are impressed by the conspicuous absence of criticism of these particular proxy sittings.
For Example Professor Dodds says this about the possibilty fo fraud in the proxy sittings:
The hypothesis of fraud, rational inference from disclosed facts, telepathy from the actual sitter, and co-incidence cannot either singly or in combination account for the results obtained ( Dodds 1962).
You can get more information on the proxy-sittings evidence for survival after death here. http://www.enformy.com/LeonardCh05.htm