"So, how did you get into QA in the first place?"
There are three classes of answers that I hear regularly: those who have a calling to test, those who fell into it, and those who aren't really into testing but can't get their preferred job.
Test Is a Fallback
These are the candidates who consider themselves "really developers, but the market is tight right now". Or else they're just out of college and looking around, with development as a default. Sometimes, they really love the idea of working for your company and will try to get in to QA with the intention of moving to development "as soon as something opens up".
If you take this candidate with a clear understanding on all sides, this can actually be a good hire. First of all, points for honesty and openness about viewpoint. Secondly, if the candidate really does end up in development, they will have some exposure to how you test and can be an advocate for QA after they've moved. Be careful to look for some ability here, though, as the desire to learn how to test well can be very limited. Lastly, if it's someone coming right out of college, give the kid a break - there are very high odds that the exposure to test is minimal at best. It may simply have not occurred to the candidate that a career in test opens him to some of the more exciting and even innovative work he can do.*
Fell Into Test
I have a soft spot in my heart for this candidate, mostly because this is how I got into QA in the first place. Let's face facts, though - testing as a profession within software development isn't one of those things you're going to hear touted in university Computer Science programs. When you get out of your average business school, there is lots of talk about being a marketer, or an accountant, or a financial analyst. When you get out of your average Computer Science program it's simply assumed that you'll be a developer, and the only real question is web developer, or database developer, or server developer. Test simply isn't on most lists. A person like this will often tell a story about being in tech support and being pretty good at figuring out what's going on, so moving into QA. Or they'll talk about an internship they happened to get and discovering that they really liked this test thing.
The best hires I've had are from this pool. There are a few flags to watch for - specifically, the talent has to be there and the technical skills need to be present. Also, if they haven't been in QA for a while you may be looking at a person who is just good at disguising that this isn't really the job they want. This type of a varied background often makes for a tester that is sensitive to the technical and business issues around the tests. The end result is often a very effective tester.
Calling to Test
These are the ones who feel like testing is a calling. Typically, they'll say things like, "I've always wanted to be a tester." or "When I was growing up, even then I was testing things." A candidate who mentions growing up a tester invariably tells one of two anecdotes. The first anecdote is about taking something apart to see how it worked, almost always a vacuum cleaner.** The second anecdote is about being introduced to something and finding a bug in it without any seeming effort. Either way, this candidate feels like they were "born to test" and have a lot of innate talent for testing (which usually means an innate talent for finding flaws).
Beware this candidate. It's possible that this will be a great hire, but all my bad hires have come from this pool. The flaw you have to watch for here is that belief in the person's innate talent - that talent may or may not exist! This type of candidate, particularly when inexperienced, also tends to be resistant to learning or instruction; why should they learn techniques and strategies when their own natural abilities will get them through? Lastly, there's the risk that this candidate simply has a strong idea of what they think you want to hear, and that's not a good sign. I want employees who are forthright, not playing the game of "please the boss".
* If you need to sell a candidate who is thinking development simply because there's no exposure to testing, then by all means do it. I'm of the firm opinion that test today is much more exciting than development (sorry, any of the devs who read this - don't kill me!). After all, who wants to get out of school and go be a junior developer implementing yet another AJAX control with a login form you'll build 50 more times in your career? Test is much more of an exciting frontier. It's where a lot of the innovation is, and wouldn't you want freer reign to solve a problem that hasn't been solved well before?
** Side note: Those poor vacuum cleaners! Why do they always get picked on? And will the next generation be telling stories about dismantling Rumbas?