I think you're referring me? No offense, but your post appears inequitable. "[tortured synonym for “know”]" — My teacher used these words. And I don't mean "know". Prognosticating a step isn't the same as KNOWING a step. I know now that GME (GameStop stock price) rocketed to $488, but I didn't prognosticate this. "under-researched" — Which posts? What do you mean? I already use two textbooks on probability. How much more research you want? Do you want me look up five textbooks or something? "overuse of images" — Fine. I admit this. But I have disabilities! Mathjax is hard for me. You can see MathJax doesn't render in many posts! But I type out paragraphs that have just text.

"bad English" — This is ungracious. English is not my first language. You can't deny that fluency in English is hard. Or else whole world is fluent! "basic failures of reading comprehension" — Honestly, I have no idea what you mean. Where's evidence? Which posts are you referring? "... really, it tends to be a painfully long list" — Again, what do you mean? List your allegations please.

@DNB, your teacher seems to have a vocabulary full of obscure English words which reference supernatural beings or supernatural knowledge of the future (soothsay, augure, fey, sibylline, vaticinate), but that vocabulary isn't suitable for use (a) in mathematics; (b) with native English speakers who haven't studied classics; (c) with non-native English speakers. Prognosticate (or its far more common synonyms predict and forecast) doesn't work the way you want it to either: it's about anticipating future events, not your own immediate actions. The straightforward way to phrase these questions is as the title of this question (How would I know) or the very similar "How should I know".

@DNB, I have given you specific feedback on each of these points on questions where they are applicable. At some point I stopped trying, since you didn't seem to be trying to incorporate the feedback into your future questions and I have better things to do with my time. But regardless, this question isn't ‘let's talk about all the things DNB does wrong in their questions’ (and I accept the blame for originally writing the question in a way that might make you and others think that it is); it's about the specific issue of asking about how to know the next step in a proof.