How to Identify a Rhetorical Question?

There are some clues you can get by studying the form: negative assertion, metaphor, marker phrases etc.

But here's a definition of a rhetorical question from Wikipedia:

A rhetorical question is a figure of speech in the form of a question that is asked in order to make a point and without the expectation of a reply... posed for the sake of encouraging its listener to consider a message or viewpoint. (Wikipedia)

I don't mean to repeat what you already know, but this is precisely what rhetorical questions are.

A rhetorical question is built deep into the context and you cannot separate one from the other. You have to be exposed to the message in the context for you to appreciate the rhetorical device.

If you really wanted to, one thing you could try is to answer the question quietly inside your head. If the combination of the rhetorical question and your answer seems silly enough in the overall context of things, then your hunch that it's a rhetorical question is very likely correct.

Ex. It's time to act! If not us, who? If not now, when? What are we waiting for?

Ex. How stupid is this new filing system we have?

Ex. A: Did you get some last night? B: Is the sky blue?

There are many kinds of rhetorical questions, those to get the listener to answer approvingly in the manner the speaker intends, others to confirm agreement. So it seems like there is very little syntactically that distinguishes a rhetorical question from sincere ones. For example, if someone asks you "Can't you do anything right?", though a bit confrontational, they may actually (but unlikely) asking for what it is that you can do right.

If spoken, a rhetorical question often has different intonation than a regular question, often intoned like a statement

However, -any- question in a speech or written discourse, with no allowance for response, must be rhetorical, by definition (there's no way to respond).

Also, English is well known for its tag-questions which are arguably always rhetorical. "You're coming with us to the movie tonight, aren't you?" (an answer to that might be expected but a positive answer is surely the expected one).

Most questions that expect an answer can be stated with a rising inflection at the end (yes, this is cultural and not universal, but fairly common in U.S. English): "Are you coming?" (with a rise on "coming").

Most rhetorical questions do not rise and may even drop in tone: "Do you really mean that?" (with a drop on "that").

This is far from foolproof, and does not translate to the written word (note my need to add a parenthetical to show this criterion), but it can often serve as a guide.