The first question you would probably expect is something along the lines of:
Here's a default installation, now remove the "Back to School" block from the home page using local.xml.
You mights also find something such as:
After installing a third-party module that modifies how product images are displayed, the product view page is missing another feature generated by a separate third-party module. What is the first step to debug this? (hint: 'depends').
The customer would like a space on the home page where they can easily modify content without having to deal with much 'code'. How would you implement this?
I personally saw all of these questions during an interview.
And then, of course, expect some questions regarding common Magento mistakes during development (clear cache, reindex, base_url mistakes, etc). If you nail everything, expect some more difficult subjects (EAV, class rewrites, updating a block via AJAX).
Depending on who you're interviewing, it might be enough to just have a good understanding of a few basic areas of Magento (layout XML, working with templates) since the first months of your employment may likely involve fixing random frontend issues on random sites. If you're relatively new to Magento and find yourself hired as a jr developer, expect to do a lot of on-the-job learning.