In the 1980s Popoff was investigated by noted skeptic James Randi. Randi worked with popular TV host Johnny Carson to expose Popoff's methods on national television, and Popoff's popularity (and viewing audience) declined considerably as a result. His ministry declared bankruptcy in 1987, though he perservered and continues preaching to the present day.
Popoff's ability to know about his audience members ailments was part of his scheme (through the power of "Ja-hee-sus") This was accomplished by wearing an in-ear radio receiver listening to his wife, off stage, read from cards that the audience filled out before the show. Although Popoff was widely discredited as a fraudulent huckster, he has recently reappeared on late night television as a preacher.
Recommended reading: The Faith Healers by James Randi