If the people you are talking to look confused and flustered by what you are saying, you are most likely using the wrong language. This is very common with my friends; where everyone would get confused once someone started to speak the wrong language. It is like a friendly warning that you need to change the language before you start to speak alien to them.
Even try to catch yourself red-handed! This will make sure you think before you speak and that you actually know what you are talking about. And by doing those two things, catching yourself should be a rather simple task.
One language per sentence
It can be easy to confuse languages when you are trying to mix the words of two different languages into the same sentences on the fly, allowing for some troublesome confusion. Try to stick with one language per sentence, such as Chinese one sentence and English the next. This should allow you to keep track of your languages better.