“When did you start? What attracted you to the position? How did you find out about the position? Did you interview a lot before you decided? Where did you interview? What type of work are you doing now? Where was your last position? Who did you work for there? Why did you leave? What was your position?”
It is pretty obvious here that you are looking for information that you can work with, for example, potential positions from their prior employer or companies where they interviewed.
Some probing questions to find out if they are completely satisfied:
“What originally interested you in the position?”
“Now that you have been there for a while, are your original reasons for accepting valid? What were they?”