Reading through the comments, I've noticed you can tell the native speakers by their scores in the word categories. A native speaker will score 20/20 in the first two bands and progressively less in the following ones. For those who have learned English as a foreign language, the scores are more evenly distributed.

So it's not uncommon to see a native English speaker totaling 90 as 20,20,19,17,14, and a foreigner reaching the same total as 18,18,18,18,18. Strangely enough, the algorithm favors the latter, because it assigns more weight to the higher-end bands.

Is this of any use? I doubt so, but it was fun.

P.S. of course a more reliable clue of nativeness is the use of "its" and "it's" interchangeably, a mistake EFL learners wouldn't do.