1St.Zed88 - on "no immediate way to measure it": you have one, and it answers your question better than anyone's signal list. Page Insights shows post reach and post engagement for each of your posts, available as soon as the post is published - from your Page's dashboard (Meta is renaming it Creator/Business Dashboard) or in Meta Business Suite under Insights > Content. So "engaging" stops being a vibe: it is the posts where the engagement relative to their reach beats your own recent average.
Two cautions so the measurement does not mislead. First, the baseline is yours, not a universal number - what counts as strong depends on your niche and audience size, so compare your posts against your posts. Second, a single post proves nothing: topic and timing change between posts, so one post beating another can be the topic, not the format. The workable version is a small deliberate series - vary one thing (format, hook, length), keep the rest similar across a handful of posts, and see which side keeps landing above your baseline. That is an experiment to watch, not a recipe, and a handful of posts can point a direction without proving a rule.
That also turns the confident signal rankings in this thread, like dinhduchoang's watch-time-first ordering, into something you can test on your own page instead of take on faith. And it beats producing volume and waiting: the waiting is real (a few posts per variant), the blind part is not.