First, Questions are in the indicative mood, not the imperative, subjunctive or optative.
Second, sometimes, not always, there are question words in the sentence like τίς, τί, πῶς, ποῦ, ποῖ, .....
Τhird, sometimes the word ἆρα (with the circumflex accent, not the word ἄρα) either starts the sentence or is the second word.
Fourth, sometimes indirect questions using words like ὅπως can be questions, but not have the question word flags.
Fifth, nothing. YOU HAVE TO FIGURE IT OUT FROM CONTEXT, as Markos said.
When you are reading Greek texts, the editors always put in punctuation. So you usually don't have to guess, unless you want to disagree with the editors. But if you are reading texts without punctuation (Some online texts strip off all the diacriticals -- accents, breathing marks, iota subscripts, and diaerseis), then you have to figure out if any given statement is a question or statement of fact. Take the following sentence...is it a question or a statement?
συ ἀγαθὸς εἶ
Louis Sorenson Λεωνίδας