I have hired a few people in my time. Not many, and I'm not an expert, but I have learned a few things. In my experience, the best programmers exhibit obvious enthusiasm in 3 specific areas.
The first is their hardware. Truly great programmers take great pride in their setup, although not in the same ways. If you can get the gleam in the eye when off-handedly talking about the latest CPU, the home network, or custom case, or skinned interface, you have found proof that they really are into what they do. For me, it's my computer. I have never bought a computer. My dad purchased my first PC in 1991, and I have upgraded it ever since. My x386 is now a P4 and soon to be a Core2Duo. Of course, I love my home network I've been fiddling with since 1998 and I can't use a stock XP installation for more than a few minutes before I have it all modified up.
The second area I borrow from Jeff Atwood of Coding Horror. Yes, again. The interviewee should have a favorite piece of code that they have created. Not usually an entire application, but a class, algorithm or routine that they are especially pleased with. Don't blame them if they can't put their finger on a single one, I have a hard time there too. Either my frequent implementations of Linked Lists, the automated Secure File transfer / Content Management solution, auto-validation in C# from custom attribute assignment, or my comic strip collector... It's especially important to me that they have some personal code they love too, not just professional.
The last area I think is important is probably the most controversial: gaming. Find out what kind of games they play, it may say a great deal about the kind of coder they are. FPS gamers tend to be very direct, task oriented, focused coders. RTS guys are more big picture, and turn-based strategy players even more so and very methodical. If you find a Civ Addict, hire him now! MMO gamers are probably good at maintenance or grind programming. And the ones that love them all will not be exceptional in any one area, but will be very versatile. That's where I lean, though I LOVE Civ; so much so that I can't leave it on my PC for long before it becomes a problem.
These generalizations are just that. Not to be taken too seriously, but they have never let me down. I have found that I cannot even consider someone who doesn't exibit much in any of the categories, or prefers XBox gaming, or doesn't own a PC (!).
There are always exception, these are just my observations.