Some questions I'ved asked, to get a feel for how much the person understands the technology he's using:
1) What are the primary differences between developing in a managed environment (C#, Java, etc) and an unmanaged one (C/C++ etc).
Some people stammer and go all glassy eyed. ("Because it's newer"). Some have some half-remembered or non-specific answers. ("Because it's safer" or "Because we get to take advantage of the .NET framework"). With others, you can see straight away that they
have experience with unmanaged code and the advantages of managed code - less worries about memory management, having runtime checks to prevent access violations and overflows, and so forth.
2) Strings. Ask people how strings are implemented in .NET, why we need the StringBuilder class and, as an extra bonus, what all these interned immutable strings are good for.
3) XML. Ask about the different ways of working with XML files in .NET. Assuming they pass the first stage, ask them when would an XmlDocument be appropriate and when an XmlReader.
All this assumes that these are even in your area of interest. Don't bother asking them about things that they don't need in their daily work.