https://newsinteractives.cbc.ca/longform/brainwashed-mkultra
I mean, just do a quick search about conspiracy theories that turned out to be true and you'll quickly see the kinds of things that have actually occurred throughout history, and how many times the crazy conspiracy theorists were right.
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.
Where do you get this idea that conspiracy theories can't be based on evidence? It seems like you've preemptively defined "conspiracy theory" to mean ungrounded bullshit. Some conspiracy theories have plenty of evidence, and some don't.
You can think of it as humid weather vs rain. That’s just a spectrum too. And yes the world is grey but when it rains I’ll be damned not to know the difference.