Talk to them about school projects or classes. I recall one candidate was in a compiler class, and told me about the compiler s/he had working. I asked 'what's a recursive descent parser'. They didn't recognize the term. I asked what grammar the language had. They didn't know. Other's show excitement, talk about having had to study things on their own because it wasn't covered in class but they really wanted to learn it, and so on. You quickly get a sense of who has a fire burning and who is going through the motions.