Who decides what's a conspiracy and what's an uncomfortable truth that powerful lobbies are effectively covering up? e.g. the broadest NSA SIG INT collection efforts, sub-concussive hits from football causing CTE, and sugar-rich "low fat" diets causing rampant diabetes were all at one point in time called baseless conspiracies by experts in these fields.

Actual evidence decides that. My general rule of thumb is, how easily supporters/detractors of an 'idea' let a third-party audit/research their findings and how they react to the results.