Gaps in my knowledge
I want to find out gaps in my knowledge of American English grammar. Which test should I take? Is only the Oxford Online Placement Test good enough for it? Or you can recommend me something else. Another approach, maybe.

Here's an online test for you
http://www.english-test.net/esl/learn/english/grammar/ei041/esl-test.php

* or can you recommend something else ?

Here's another test :)
http://www.english-test.net/esl/english-language-proficiency-test.html