"To know that you know what you know,
and that you do not know what you do not know,
that is true knowledge."
This Skills Assesment site has various tests to find out what you know about a range of Microsoft's products, including the .NET Framework of course. Since the current certification exams don't show what you scored on the different areas, these tests are a good indication of where you should focus a bit more. After taking the tests, you even get advice as to books and resources where you can read up.
Good business for training companies I would imagine.
I scored 80% on the first test, so there's plenty for me to learn (phew)... and to think that I knew it all ;-)