Innocent until proven guilty, says I; and in this case guilt can only be proven in hindsight if-when things go off the cliff.
Beyond a reasonable doubt. Innocent until proven guilty
beyond a reasonable doubt. We don't need a smoking gun, we just need enough reason to believe that no
reasonable doubts remain. That doesn't mean there can't be any doubts, nor even doubts that are based on realistic possibilities (that is, ignoring ridiculous options like "fairies did it" or "he was actually killed by a meteorite impact"). Meteor impacts do in fact happen, for example, and it is possible that a murder trial where no body was found could've been the result of a flash flood that occurred after the (alleged) victim's disappearance. Is that a reasonable doubt, especially when coupled with evidence that the defendant secretly obtained a weapon and then disposed of it afterward, took out a life insurance policy on the victim mere months beforehand, and has extensive Internet search records about how to dispose of bodies in undetectable ways? That's what juries are for--and in many cases, they decide no, the "was just missing, died in a flash flood that happened while missing" explanation isn't reasonable, nor are various other realistic but unlikely scenarios.
That's the part that's extremely important here, and why (for example) circumstantial evidence, despite the huge stigma against it in fiction, is actually extraordinarily important in real legal proceedings.
As with an enormous amount of this stuff, you always presume that the player already knows you extremely well and would have an enormous backlog of reasons to trust you. Why? The
vast majority of GMs out there don't have that pre-established background. Even if most of a GM's players are their friends, you never know how someone will handle a given responsibility until you see it in action.
Responding to someone who has a
legitimate concern with "I literally can't explain it to you" or "You just
have to trust me for several months" isn't going to fly in the vast majority of cases. It's that simple. You are advising people do something that is directly counterproductive to the goal of establishing player trust that the game will be run fairly etc.