I dislike the term "multitasking". Anyone can say they multitask - by virtue of the fact that a human can breathe, digest, and walk at the same time, we're all multitaskers. In reviewing resumes, recruiters look for content - specific information that gives some kind of inkling as to how the job candidate spends his or her days. Then we use a term like "multitasking" to describe the position we are advertising.
A few years ago, a study came out stating the human brain isn't meant to multitask in the way the term is commonly defined. We are supposed to focus on one concept, then another, not two or more simultaneously. Personally, I think it's an evolutionary step - our environment and our jobs require that we filter out extraneous noise, evaluate and accept or reject incoming data while simultaneously completing tasks. We're bombarded with information, and at some point we had to learn how to take it in without allowing it to consume the focus of our thoughts.
From my view, a better way to find out if a person has the ability to successfully manage incoming data, noise pollution, and the tasks of the job is to focus on the ability to prioritize and change focus quickly. A question from a manager may not be more important than the task at hand, however it has a higher priority and must be addressed as expeditiously as possible. A good interview question to learn about this ability might be "Tell me about a time when you were working on an important project with a tight deadline, and your manager asked you to answer a question. What did you do?" For an entry-level candidate, the best answer is "I asked my manager for direction as to which was higher priority, the question or my project". A mid-career worker would probably say "I answered the question and returned to my project, and met the deadline". A high-level employee might respond "I delegated the question to a member of my staff who had the data readily available, and followed up with my manager to make sure the information was delivered." These are all the right answer, and appropriate responses.
Sadly, the need for prioritizing is simply lost on some people. In terms of skills, this is a trainable area of focus for those of us who mentor, supervise, or coach. For some people, this is an innate ability - they know that you don't open your e-mail based on order of receipt, but instead on order of importance. (I'll save my diatribe regarding non-urgent items marked as urgent for another day - but that's coming, too). There are many books and articles available with great suggestions for learning to prioritize.
From my perspective, here is the best question to ask when trying to decide which item is most important - who do you LEAST want calling your boss to follow up? There's the first priority of the day.