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.