You can use System.Environment.Version
to the get the version of the CLR; how do you get the versions of C# and of the .NET or Mono framework currently being used?
EDIT: I am NOT asking how to find whether you are using Mono or .NET. (The very first version of my question did ask that as a secondary question, but I deleted that almost immediately when I saw that question answered elsewhere.) The questions here are (1) how to figure out what version of C# you are working with, and (2) what version of .NET you have (as opposed to the version of the CLR, which isn't the same thing).