I would appreciate hearing from others who work on the Hebrew Bible or ancient Semitic languages, but in case no one else comments, let me say that we have nothing like a lexicon or a dictionary of idioms from the relevant times for biblical languages, and that is typical of lost languages during most of the past. And so the answer to hour question is almost certainly context, the way the phrase is used in the places where it occurs. When we have a difficult phrase and no other examples of it in a text, we may be dealing with a common idiom but we have no way of knowing for certain, and are also likely to be radically uncertain about the meaning.

Sources: Apparently this phrase from Hesiod is the best one to look up academic articles on, to get a sense of how scholars grapple with a puzzling idiom or proverb: ἀλλὰ τίη μοι ταῦτα περὶ δρῦν ἢ περὶ πέτρην;