How to interview undergraduate candidates for research assistants?
My advisor, bless his heart, entrusted in me the responsibility of interviewing and selecting undergraduate students to work in our research lab while still being a graduate student myself.  Sometimes it's difficult to determine what a person is really like just from a few minutes speaking with them.  I found a few particular questions to be particularly helpful:  


Ask them what their goals are for the future.  Most undergraduates give very general answers because they are not sure what they want.  A highly motivated student knows what they want, and are likely to link their goals as part of the reason why they want this particular research job opportunity.  
Ask them to describe in detail a difficult problem that they had to overcome, and explain how they overcame it.  This is a question they typically anticipate, but it's the manner in which they answer that matters...  Observe how well they put together their answer, how long it takes them, and how well they articulate it and its relevance to the research job that they're applying for.
Ask them about their greatest strengths, and greatest weaknesses.  In particular, examine how they explain how their weaknesses may affect their job and how they intend to overcome these weaknesses.  A strong candidate will not only be self aware and honest, but will seek ways to overcome these deficiencies.