Given human nature and all the people who have ever lived, it seems likely that there have been millions of conspiracies spread unevenly over time, with most of them happenning recently. This means that we cannot issue a blanket dismissal of every claim to have discoverred one, even though it also seems that the majority of such claims are bogus. Thus the scorecard seems litterred with high counts of both false positives (bogus discoveries) and false negatives (undetected conspiracies).
Examining an example may provide illumination. Consider the claim that the moon landing in 1969 was faked. So many of the claims are focussed on the video images and some notion that producing fake images was some how easier than the actual effort could have been. While it is lots of fun to poke holes in these claims, it seems to be only a distraction with no hope of helping the poor fools who peddle them.
A much more complete rebuttal comes from going back to first principles. A conspiracy, by its very nature, is about secrecy, and not just at the time of the event, but forever. Secrecy is so very hard to maintain, and every additional person involved makes it harder. The moon landing event involved tens of thousands of people, regardless of whether it was real or faked; not quite enough to dump the claim yet.
Where the claim completely falls apart is in examining the audience for the event. Yes, the millions of viewers play a role, but most of them are just insignificant viewers. To understand, we need to look at the motivation for both the read event, if it happenned, and the illusion, if we were to suppose it was faked. The ordinary viewers in front of a family TV in the USA are a big part of the numbers but only a tiny part of that motivation. The principle target of the show was the masses of people around the world involved in the political competition between the USA and the International Communists. The family viewers may be easily duped (and are regularly), but the science and engineering communities around the world are much more discerning.
In this analysis, the video is actually relatively unimportant. The crux of the situation is the source of the radio (TV is just a special format of radio) transmission. Radio direction finding is technology that was already well understood in the 1930s. The spheroid shape of the earth, together with witnesses scatterred around the globe, means that faking the transmission would be detected immediately; do not doubt that the USSR would have pointed out such a glaring contradiction if the transmission had not been from the moon.
In summary, lunar landing deniers expect us to accept that tens of thousands of people worked for the USA to produce a fake video (along with the appearance of a functional rocket) that depends on a robot lander to get to the moon intact and operational, carrying either the video or a radio relay, in order to persuade the rest of the world that the USA is superior to USSR, and to reject International Communism. It seems so much more likely that the USA would risk scores of astronaut lives doing the real thing.