My approach is twofold:
I) Testing problem solving and other "traditional" consulting skills. This will be based on my 18-years experience in the sector and hundreds of recruiting interviews in several geographies.
II) Helping the candidate assessing his/her long-term potential success as a consultant and real motivation to undertake a career in the sector. I think this should go beyond the sheer recruiting process (go/no go of the interview), but rather involve a very personal reflection.