|Test your vocabulary at testyourvocab.com|
Now apparently this doesn't include the words you 'sort of know', but words you definitely know. As in you can give at least one definition of them accurately.
Well if you'd like a rough idea of the extent of your vocabulary and would also like to help out a research project at the same time then head on over to testyourvocab.com and take their very short test.
You can even bookmark your results to share (show off) with friends and family.
Here's a short description of the project from their site:
"TestYourVocab.com is part of an independent American-Brazilian research project to measure vocabulary sizes according to age and education, and particularly to compare native learning rates with foreign language classroom learning rates."