My approach has just been to have a conversation, and try to move it along naturally. What do you think of concepts? What useful STL did they add? Why did you go with this design? Ask me something.
If the person doesn't really have the level of interest required, they will run out of things to say. Like a cave left unexplored. The good candidates will go down all the caves with you, even if they don't quite know what's there.
In particular, the times I've had dud hires it was not possible to know from the standard fare of coding questions and IQ questions. When I was doing that obviously the people I hired passed. But for instance one guy really didn't want to be a programmer, he just knew how to answer the questions. He ended up stalling on his first assignment, and eventually decided to change careers. Another dud also lacked motivation, perhaps because he was asked to do something not quite in his area to begin with. He ended up not doing anything at all.
Apart from those two, dozens of others have turned out ok. I never ran into that guy who couldn't do FizzBuzz, despite not asking. I also don't think it was all that bad to let go of the duds within a quarter, they never got so ingrained in the team that we depended on them, and they could plausibly move on to another job without mentioning us on their CV.