Spoken communication has something that written communication can never attain perfectly: inflection. A little lift in tone can give emphasis, tell when someone is being snarky, and signify a question. Writing doesn’t have that luxury—and I think the two-dimensional nature causes some stumbling with question marks.
What a Question Is
First of all, a question is a direct inquiry asked of someone or something. This doesn’t mean a reply is expected (e.g., rhetorical questions); it just means that the question has to be implicit and not framed by something else. You usually know a question by the “question words” that start the sentence (e.g., did, do, does, where, who, what, why, are, will, can, etc.). But no signal word need be used—especially informally—as long as a request is made.
Have you seen him pull out his Sharpie when he scores a touchdown?
This make sense to you?
One thing to watch out for is that question words don’t necessarily mean that something is a question. The title of this section is “What a Question Is”; this is not a request, but a statement of fact. Always check the motivation of the phrase or sentence to see if something is being requested.
Many times people frame questions as part of a larger thought or sentence. In such cases, you don’t use a question mark.
He asked me how many times I got a gutter ball.
In this case, there was a question asked of the narrator, but the narrator is not asking a question directly.
A trend that I’ve noticed is the use of question marks when giving a command, as if the question mark softens the request for action. Here’s an example:
Guess how many gumballs are in this gigantic container?
That is, however, wrong (as was my guess). If a sentence begins with a verb telling the audience to do something (an action, even mental), there won’t be a question mark. Commands aren’t requesting an answer; they’re directing.
Another common misplaced question mark occurs in wonder statements. The person speaking or writing ponders the significance of some deep subject and gives a statement of philosophical circumspection.
I wonder how many rivets there are in my blue jeans.
Notice that this is a mental action and not a direct request for information.