Interviewing is difficult for many organizations. Often they don't have a consistent process, and very often the people that do initial screening don't know much about what a DBA does. Many of the somewhat trivial questions I see on Internet lists (what's a clustered index?) are suited for someone getting started in the business, not for hiring an experienced DBA.
Sure, an experienced person ought to be able to answer those questions, but the real world often requires more nuanced questions that relate to the situations we find ourselves in. I found a list from Joey D'antoni recently, which has lots of open ended questions, designed to allow someone to talk about what they know and think, giving the interviewer the chance to gauge their level of expertise.
I especially like the questions around tuning and configuration. Tell me what you've done in the past and why. That way someone can tell you a story about their experience and then relate this to practice solutions. There is no good way to talk about these situations if you haven't lived them. You might read someone's blog, but a few follow-up questions on details will show whether you really understand the solutions or not.
Of course, you need a good interviewer that knows something about these topics to understand whether the answers make sense or not. An HR person or manager isn't likely going to be able to judge how a candidate performs. While a developer or sysadmin might be able to gauge whether the person has spent time with SQL Server, if you need a really experienced person, you need someone equally experienced to interview them. That can be a challenge for some companies.
Hiring is difficult, but with more junior or intermediate positions, it might not matter as much. Evaluate whether the person has some knowledge and fits with your team. Can they work with others, which isn't the same as just agreeing with them. Maybe more importantly, can they learn. Do they learn from your questioning, or have they shown they are learning.
For more senior people, references and networking matter a lot. If others recommend them (or don't), that says a lot. It's good to initially decide if this person is a good fit, and then engage someone like Joey or Glenn Berry (or Tim Mitchell in the BI space) that might provide a better evaluation of your short listed candidates.