(Drawing a deep breath). Here is how Corboy imagines, just imagines how an ambitious faith healer goes about selecting malleable persons.
Just an imagined scenario.
An operator who is experienced with doing trance induction knows there are visible physical signs that audience members are at different stages of trance induction.
The experienced trance technician and his or her assistants can identify audience members most susceptible to trance, then learn their names and get background information.
From there its easy to tell the mark that he or she is special.
Do some Googling. Look up physical signs/indicators of trance or indicators of stages of trance induction. Try a variety of search terms.
See what comes up.