Michael Prescott has written an informative series of posts on some questionable mediumship here, here and here.
He will also be appearing to discuss this topic on Marcel Cairo's Afterlife FM internet radio show next Tuesday at 7:00 PM EDT / 4:00 PDT. Marcel is a mental medium, who himself believes that most or all physical mediums are frauds.
Michael is one of the the world's foremost amateur experts on the history of the scientific investigation of mediumship, the evidential cases, and the many frauds. It should be an entertaining and interesting show.