How can you distinguish good English from bad English?
Literal
Good English [and-と], bad English [topic-は] how-by distinguish can [question-か].
~はどのようにして~か ('how does one X?') is a standard question-of-method structure. どのようにして ('how, in what manner') is a formal variant of どうやって — the te-form of やる ('to do'). 区別する ('to distinguish, differentiate') takes を on the items being distinguished.