The same way a parent figures out their child's deviant behavior?
From out their *****.
They good guesses
They don't. There are certain emotions and feelings all people have and a lot of what they would say is general stuff. Psychics are frauds.
There are advanced social engineering tricks that you can master and get information out of a person without them knowing.
She had to have. Even indirectly. Clever sideline prompts get people to say anything without realizing.