Question tags: the tune decides whether you are asking or checking
The same question tag can check or genuinely ask, and only the tune of the voice tells the listener which.
You've met before, haven't you?
The model sentence can be said with two different tunes, and the tune changes what the sentence does. With the voice falling on the tag, the speaker already believes the two people have met and is inviting the listener to confirm it: the tag is a small piece of politeness, not a question. With the voice rising on the tag, the speaker genuinely does not know and is asking. Both versions are written identically. In conversation, native listeners hear the difference instantly and answer accordingly: a falling tag gets a brief agreement, a rising tag gets real information. At this level, controlling the tune is what separates a speaker who sounds sure of the ground from one who seems to be checking everything.
Most learners rise on every tag, because in many languages a rising tune is simply what a question sounds like. In English that habit has a cost: a speaker who rises on all their tags sounds permanently unsure, and a listener may feel interrogated by what were meant to be friendly remarks. The remark about the weather in the first example is not a request for information; it needs the falling tune. Practise the two tunes on the same sentence until you can choose one deliberately, then listen for which one native speakers use in small talk. It is nearly always the fall.