With programmers, the single easiest way to identify good candidates (in my experience) is sheer interest in what they do / desire to learn. This is a learn everyday field and if you're interested in what you're doing, you're going to do a lot better at it. It's hard to apply yourself mentally to something that you don't have a good level of interest in. Given that it's a learn everyday field, people with that level of interest will realistically be able to learn anything they need to do solve the problem you're hiring them to solve.

The only real differentiating factor is your tolerance for ramp up time. I expect a programmer to be able to pick up a new language or database within a couple of weeks (tops) in most cases. If I'm hiring full time, that's something I'll tolerate. If I'm hiring a contractor, I'm going to be uneasy about paying high hourly rates for him to learn the job.

The single most effective way that I've found to interview for "interest" is to just get them talking about something they've done before and ask them to go deep into the details. You get everything you need from watching somebody talk, with a smile on their face, about how they solved some problem in a creative way that makes them show some pride. Doesn't really matter what the problem was, if it was a business problem, code problem or hardware problem. The important thing is the level of attention to detail in addressing it.

I've been using this technique for about 8 years now and while I don't make it the exclusive criteria for hiring, every person I've ever hired who has passed that part has ended up in my "great hire" category.