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.
Here are some good question words to look for:
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.