Hi, there are a lot of Germans teaching English here. With plenty of practice it's not too hard for Germans to acquire a near accent-free pronunciation, which is what you will need. Compare recordings of your own voice speaking English with recordings of the same text spoken by a native English speaker and see how close you are. If you have an Abitur or a degree in English, it's just possible that your grammar may be even better than that of an average native speaker. For teaching in mainstream schools from grades & to 12 your vocabulary will probably already be more than adequate.
If you really want to self test yourself, get some of the books and practice tests for IELTS (British English) and /or TOEFL (American English).
Be very careful how you choose the course provider in Thailand.