The kind that comes from more than a single person, for a start. An unequivocal sign of a conspiracy (like an actual explosive attached to a support).

Failing that, a report free of clear signs of confusion (like the aforementioned confusion at 4:39). Reports of explosions from people actually familiar with explosions, and/or experience and a track record of cool under threat ("a boiler guy" and bureaucrat don't qualify, without more of a evidence). A witness who hasn't changed his story back and forth. Etcetera.