The simple two-part question to identify when a meeting is worthwhile and what the scope of the meeting should be (and what should be in other channels): Is simultaneous, interactive, many-to-many communication necessary, and why, specifically, is it necessary? (A followup, to make sure that people are ready to have the meeting -- another common problem that makes meetings a waste of time and results in inappropriate things being done in the meeting venue -- is to ask: "what needs to happen first so that people are ready to engage in that many-to-many interaction"?)

Lots of meetings, IME, are held by one person to gather information from many people or to distribute information from one person to many people -- these kind of one-to-many or many-to-one scenarios are the easiest thing to see doesn't require a meeting (many-to-one being the more inefficient.) There's even some cases of many-to-many communication where there isn't any real need for interactivity. And plenty of cases where a meeting that is held for a many-to-many interactive purpose spends much of its time doing top-down, one-to-many communication for much of the meeting because something that ought to have been distributed to be reviewed by participants to be ready for a productive meeting was instead distributed for the first time in the meeting, wasting most of the meeting time.