That information isn't public. I'm a mod here (in case that wasn't clear), so I looked at the private data.

Edit: if you meant what you can tell from public information: (1) sudden drops in rank, as walterbell points out downthread—(edit: but it's important to know that sudden drops in rank can also be because of the flamewar detector, which is unrelated to flags), and/or (2) the [flagged] annotation, which appears when flags outweigh upvotes by a certain threshold.

I assume they were asking how does a non-mod tell if it's flagged so they can report it to you? (aka when it says [flagged] next to it or something, or drops off the front page rather quickly, etc)

Possibly from abrupt drop in rank?

http://hnrankings.info/27307977/

Thanks! Added above.

Oh I see. It seems that it would be difficult to report a big drop in rank unless you’re frequenting tho.

I guess I should turn on the show dead to see flagged posts

[flagged] and [dead] aren't the same, but if enough flags occur relative to upvotes, the thread will eventually also become [dead] - however, not if there's an active discussion already. So 'showdead' might not do much for you in this specific situation.