Dr. Mounce wrote on his website (also on Zondervan’s Blog) on how to determine whether a sentence is a question, but he missed probably the most important and most obvious way of determining the illocutionary force of a sentence:

If the sentence in question begins with a question word, it’s a question.

I promise.

Here are some good question words to look for:

• τίς
• ποῦ
• πῶς
• ποῖος
• πόσος

There are others, of course, but these ones are pretty common. They’re also the one that I was able to think of without going to a dictionary. If your sentence begins with one of these, it’s a safe bet it’s a question.