The quick and easy answer is, "Look at the punctuation." If it is a ";" in the Greek text, the editor is telling you that he (or she) thinks the sentence is a question.
But of course the punctuation is not original with the text, so the punctuation is just opinion; although, considering the competence of the editors of our Greek text, it is an opinion worth paying attention to.
The real answer is, "Context." What fits the author's intent? There is no change to the form of the Greek verb when it is asking a question (except see below). So you have to ask yourself what fits the flow of the discussion. However, there are three clues that the writer is asking a question.