[sampo]

> For example English has according to the Oxford dictionary 171,476 words in current use excluding inflections, and several technical and regional vocabularies.

Here is a website which questions you with some random sample of words from an English dictionary, mixed with randomly generated non-words. Then it estimates the percentage of English words you know.

http://vocabulary.ugent.be/wordtest/start

I am a non-native speaker, and I have scored in the 77% to 89% range, when doing this test several times.

[tempestn]

I'm curious: did you only answer yes to the words whose meanings you knew, or to anything that you knew was indeed a word? There were some that were pretty obviously words, but I wasn't certain the exact meaning (although I could guess), so I answered no. Ended up with 77% (as a native speaker). Apparently average for native speakers is 67%, so 77-89 as a non-native speaker sounds really good.

[wingerlang]

I just did it, and I answered yes to words I knew, or knew that were actual words but I didn't know the exact meaning of. Like Argon, I know it is something related to chemistry but I don't actually know what it is. Some words were compound words which I am not sure would be in a dictionary, but still valid words.

I got 73% and I didn't say 'yes' to any fake words.

73% is apparently "This is a high level for a native speaker."