I took Ghent University's vocabulary test, and according to the results, I know 83% of the English words.
The Minnesotan took the test and got a 94%. He said:
The test does not require you to define or spell words. Instead you are asked to determine whether an entry ("glyph," "moktam," "macrophage," "wookel" etc) is or is not a word in the English language.
There seemed to be about a hundred words in the test, which you can proceed through at your own pace, and you can retake it if you wish (with a different group of words on the retest). At the end you can review your errors and see the definitions of the words you missed.