In this world there are real conspiracies. Tobacco executives hid the bad effects. One of G.W Bush's ancestors tried to take over through the US government just before WWII. US doctors gave black men STDs. North Korea News services, well, they just are one giant conspiracy.
But some people are lunatics and believe obviously false conspiracies.
So, how do you tell the sane from the insane?
1) Sane people realize that amateurs are NOT smarter than all the experts. If you can think of it, then the experts have as well, and they have looked into it. If they don't think your idea is valid, it isn't (unless they are 'in on it').
2) Sane people realize that if in fact all the experts are 'in on the conspiracy', then that means the conspiracy has the resources to kidnap squealers. See Russia, North Korea, etc. So it's a good idea to keep your conspiracy to yourself.
3) Sane people also realize that if you disagree with the far majority of the experts and publicly talk about it and are not missing, then no conspiracy exists. The whole point of a top secret conspiracy is to stop people from telling the truth. If they let you tell the truth then 'they' don't exist. Duh.
Consider this simple test - has your President murdered anyone? If you make a serious accusation against him, will you be killed? Not in the US because our President has not done so. But I do not suggest trying that in North Korea.