Jay asked, “How do you know that this sentence is interrogative?” (speaking of Gal 1:10). I love it when I get question that is so basic that I have forgotten to deal with it. Good question.
The passage reads, “Am I now trying to win the approval of human beings, or of God? ( Ἄρτι γὰρ ἀνθρώπους πείθω ἢ τὸν θεόν; ) Or am I trying to please people? ” ( ζητῶ ἀνθρώποις ἀρέσκειν; ) The final ; is the way the editors of the Greek text tell you they think it is a question, but punctuation is later than the text and hence not inspired. And there is no special form of the verb to indicate that it is an interrogative (except hortatory subjunctives).
Sometimes there are clues with the use of negations. A sentence expecting an affirmative answer is prefaced with a οὐ. One expecting a negative answer, by μή. Of course, most uses of οὐ(κ) simply are a negation. But there are some exceptions. “Do all have gifts of healing?” (μὴ πάντες χαρίσματα ἔχουσιν ἰαμάτων;, 1 Cor 12:30). Or more properly translated, “Not all have gifts of healing, do they?”
The answer is simply, context. Is there a way to make sense of v 10 as an indicative? I don’t think so. Does it make sense as a question? If so, it probably is an interrogative.
Every major translation views this as a question, except for the NLT, which so drastically changes the verse that one wonders if this truly is translation. Under inspiration Paul asked two questions, expecting the Galatians to answer. But the NLT says, “Obviously, I’m not trying to win the approval of people, but of God. If pleasing people were my goal, I would not be Christ’s servant.” In other words, the answer is “No” in both cases, but the NLT has removed the need for the Galatians to confess the truth about Paul. That’s not a good translation decision.