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?