By using this word you already presumed the likelihood of it being true is very low, so why do you at the same insist the likelihood of it must be high?
If you believe there are conspiracy theories that turned out to be true then maybe they weren’t actually conspiracy theories but instead theories held by a group of people based on evidence (evident possibly just to them) that turned out to be true.
If your question is how do we discern between the two without the benefit of hindsight I’m afraid the answer is in the negative, we can’t, unless you want to just give undue probability of events occurring based on how you feel on one day instead of what evidence you have been provided.