According to wikipedia:
A conspiracy theory is an explanation of an event or situation that invokes a conspiracy—generally one involving an illegal or harmful act supposedly carried out by government or other powerful actors—without credible evidence.
Conspiracies happened in the past (e.g. Assassination of Julius Caesar ) happened in the recent times Gulf of Tonkin Incident, Nayirah and will continue to happen. These last two are no longer considered "theories" because they have been validated by mainstream media and reputed historians.
It is that "credible evidence" what turns a conspiracy theory into a real conspiracy. Keep in mind that not always scientific consensus makes something credible, take for example creationism. (38% of the Americans still believe in it) and climate denial (34% of Americans think scientists seriously disagree about climate change).
Also keep in mind that people have trouble accepting the truth when it comes to events which are hard to accept because the mind is not efficient at looking for the truth when you are emotionally attached to a result or idea. When this happens you may get mental phenomena such us motivated reasoning, confirmation bias or cognitive dissonance that prevent you from having an unbiased research. The more you want things turning out the way you want the harder it gets.
I'm afraid that there is no an easy way to dismiss "conspiracy theories" as fake all the time unless you do your homework or expect those ideas to be accepted or rejected in time by popular culture. Sometimes the truth is told when it is no longer relevant.
"The truth is not for all men, but only for those who seek it." Ayn Rand
"A man is his own easiest dupe, for what he wishes to be true he generally believes to be true." Demosthenes 384-322 BC
