You ask a great question because it's not easy. What I would often do is give the same assignment to each finalist.How do you demonstrate work ethic in an interview? Outside of you’re resume/credentials there isn’t much of a way to tell as an applicant can say anything they want right?
For example, I might let them know before the interview they will be asked to give a fairly brief presentation on a particular topic. When you give 3-5 different finalists the exact same assignment, it's not difficult to determine which persons put forth the greatest amount of effort in preparing.
I also tried to ask numerous questions to identify how much they knew about Iowa State and our department in particular. It helped me assess which ones really prepared well for the interview and which ones didn't.