Because of this and this and this, you should take this.
I did. I received 100% on the Fourth Grade test, 100% on the Eight Grade test, and 80% on the Twelfth Grade test, which was highly disappointing to me. (You can view more of the questions here.) I got one question wrong, and now I will have to go back and try to close that gap in my knowledge.
There are a whole bunch of tests on this site, and though I cannot spend the whole day taking them, I am certainly going to go back and do more, if only to identify what I don't know. It's easy for us to look at someone else and say they got it wrong (and, this is also appropriate, I would argue, when that someone wants to be speaking for America or making laws for America or wants to be taken as an expert in a field), but we should question our own knowledge, too.