> What topics are academic? Which are not?
Topics which discuss general ideas which are logically (or at least rhetorically) decidable are academic. Topics which discuss particular individuals and are matters of taste are not.
> Which questions are fair game for academic inquiry?
cf supra
> Which are not?
cf supra
> How do we know ahead of time what will be useful and what won't be?
If a topic falls under an existing academic discipline, it will likely be useful. As stated above, matters of taste are unlikely to be useful. (possibly, if one has reason to believe one can capture a global distribution of tastes? or provide a theory of changes in distributions of tastes, etc.)
> Is the question above this line academically useful?
Useful for a "philosophy of academics" discipline, probably not so useful for any non-meta discipline. Note, however, that discipline's core questions will change over time, as one subfield gets overworked and new subfields are discovered.