"To know that you know what you know,
and that you do not know what you do not know,
that is true knowledge."
— Confucius

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 ;-)