Did you ever think about how many words you know? Well, here is a way to find out. Not very scientific, but what the hey.
First, get a dictionary. Second, count the words in the dictionary. Huh, hard you say? Not really. Do this: Count the number of entries on a page. Take a couple of samples and get an average. Then multiply that number by the number of pages. The result will be close to the number of words in the dictionary. Third, and this is the fun part, look at the first word on the page. If you know the meaning give yourself a check. If you don't then give yourself an x. Fourth, repeat this 100 times. If you get 50 right, then you may know about half the words in the dictionary. The more times you repeat this, say 1000 times, the more accurate this will be. The % you get will probably be on the high side, since you may come across some words that are stems of other extended words. But that's okay.