Try this test.
Of course, tests like this will only tell you how good your grammar is. Having said that, it's a very good test. It will give you a good idea of your CEF (Common European Framework) level, as far as grammar is concerned. It is specifically designed for German speakers of English and can be taken by all levels - beginner to advanced. However, it highlights those areas of English grammar that challenge German B2 speakers of English who wish to make the difficult step to C1/2 (native speaker level). Give it a try and see how you get on.
What is CEF?
The Common European Framework is the most widely accepted way of assessing levels of English as a second language. The common abbreviation is either CEF or CEFR. This framework was compiled by the Council of Europe between 1989- 1996 in order to provide a common reference point for all the testing and banding systems offered by various language training organisations internationally. These many different banding and testing systems still exist, but thankfully most now accept the need to relate their system to CEF.
The advantage - and disadvantage - of CEF is its simplicity. There are 3 categories which in turn are each sub-divided: A1/2 (beginner/elementary) B1/B2 (pre-intermediate/intermediate) C1/C2 (advanced/native speaker standard). Unfortunately, there is no definitive version of what these categories specifically mean, and many, including myself, find the categories far too general. (Trying to get academics and trainers to agree about this sort of thing, is challenging.) However, there is a broad consensus. A summary in German can be viewed by clicking the link below:
One comprehensive way of finding out your CEF level in reading, writing, speaking and listening is to take a test offered by one of the international educational testing services. See here for more information.