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.
1. If the verb is a subjunctive, it might be a "deliberative subjunctive." These are first person forms, usually plural, and often near the beginning of the sentence. "Deliberative" questions do not have "Yes" or "No" answers, but rather are designed for the hearer to think over, to deliberate, before answering. Jesus tells his disciples in the Sermon on the Mount not to bother themselves with questions like, "What should we eat?" or "What should we wear?" Note how we translated this into English, by adding the helping verb "should" to express the deliberative question in Greek.
2. The second indication that a sentence might be a question is if it is preceded with an οὐ. This is the Greek way of indicating that the expected answer is "yes." The disciples ask Jesus, “Master, do you not (οὐ) care that we are perishing?” (Mark 4:38). We have the same type of construction n English, although it is more emphatic and less nuanced. "You want to visit your grandparents, don't you?"
3. The third way Greek indicates that the sentence is a question is to begin with a μη. This is just like using ου except that the expected answer is "no." "All don't have the gift of tongues, do they?" is Paul's question to the Corinthians in chapter 12. Again, the Greek is so much more nuanced, more gentle than the English, that the expected answer rarely gets into the translation.
At the end of the day, context is still king. Having a sensitivity to the flow of the discussion and what the author is saying is the ultimate clue as to whether or not a sentence is a question.
I usually try to end a blog with moving into spiritual application of what I am discussing, but I am drawing a blank this morning. So I will comment on how much I love the digital age. I am riding in a car with my wife, traveling to Seattle to watch the Seahawks play, typing on my iPad. How cool is that?
What is my expected answer?
William D. [Bill] Mounce posts about the Greek language, exegesis, and related topics at Koinonia. He is the author of numerous books, including the bestselling Basics of Biblical Greek, and is the general editor for Mounce's Complete Expository Dictionary of the Old and New Testament Words. He served as the New Testament chair of the English Standard Version Bible translation, and is currently on the Committee for Bible Translation for the NIV. Learn more and visit Bill's blog (co-authored with scholar and his father Bob Mounce) at www.billmounce.com.