1, Ask questions without a stardard answer. Instead, observe how they answer the questions.
I used to ask this question: How often do you use Linux command line? Each of them answered "everyday". Then I said, tell me 5 commands that you usually used. Believe it or not, some of them struggled a lot to get 5 commands out of their brains!
2, Ask the interviewees to bring 5 pages of source code from their previous projects. Review the coding style, ask them questions from what you see.
3, Present a snippet of code. Ask the interviewee to a) explain what it does; b) comment on its quality.