Hello,
I believe it is gauged by taming the art of interviewing.
By doing a Chronological Interview (i.e by getting into college years and finding more about each and ever phase of the candidates career choices from there) - but make sure - you stick only to the "Career Choices". Never have a appearance on your face if something wierd happened in between (i.e dont react). Just act liek like - that would be the most normal thing that could happen - in this way, you get to know more.
I would start by "Would you please give me a brief rundown on your college years, particularly events that might have affected later career decisions"
I would also ask them their expectations from each job, the responsibilities, and accountabilities, what they found upon entering the job, major challenges, strengths and weak, and particularly teh circumnstances in which they left each job.
This will help you gauge more on how the person racted to certain phases in his life, and how he managed the growth path.
And about future plans:
1. What are you looking for in your next job.
2. How does this opportunity square with your ideal position? What do you view as opportunities and advantages as well as risks and disadvantages in joining us?