Nick Halstead has posted part three of his series looking at interviewing programmers (here's part one and part two) with more of a focus on the details of the individual including their knowledge of code and general abilities.

In the murky past I wrote two article about interviewing programmers. The first covered the basics of interviewing itself and gave (hopefully) a good grounding for those who have never had to interview before. The second was all about the kind of testing you can do to filter out candidates before you commit to doing a full interview. [...] At the time I then promised to write a third section on what types of tests to use during the interview. Well I finally got round to finishing it off.

This time, he suggests things like specific language testing for your company's language of choice, an IQ test, seeing how they "pseudo code" and some psychometric testing. Basically, this boils down to measuring both the individuals potential and how good they will be in the position you're interviewing for.