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I wonder what if there is a site that helps confused users to know if their not-yet posted questions is on-topic or off-topic in their site of interest.
I mean if they can't find it out from reading the guidelines and meta posts.
Such site(s) already exist: you can ask if a specific question is on-topic on each site's own Meta. A recent example is here. Such questions are often tagged [scope] or [on-topic].
If you're not sure what site to post on, you can post a site recommendation question on Meta Stack Exchange.