In response to my previous post, I thought it was interesting to tackle what it actually means (in my opinion) for something to be credible.
The dictionary defines credibility as "Worthy of confidence; reliable." -- this means credibility is subjective because we each decide for ourselves what is "worthy of confidence". My personal standard is pretty high.
In my opinion, things are credible when they can be independently verified..
Example: Can you verify that 17 school buses were stolen in Houston? Sure.. call the Houston police, they'll tell you (don't trust them? I suspect you could call the victims themselves!). Can you verify that the air force is planning some sort of stand-down on the 14th? Call them and ask (I guess they could be lying -- but it is a fact that they CLAIM there is a stand-down). Can you verify Hillary Clinton and Dennis Milligan made the statements? It's on video.. you can see it for yourself. Can you verify that a black beard is a sign of Jihad? Absolutely.. talk to a Muslim Cleric, or study the ancient religious texts yourself.
See the pattern? You can know for YOURSELF if these things are true.
By this standard, credibility slips when items cannot be verified at the source. As soon as we use words like "my sources tell me", we've broken the chain of credibility, unless I can verify your sources. The problem is that conspiracy theorists will rarely let you verify their sources. There are a few exceptions (like Dr. Jones and the great work he's done with his 9/11 research). The more a conspiracy theory has verifiable subtext, the more credible it becomes. Unfortunately, once you can verify more pieces of the theory for yourself, it becomes less of a conspiracy theory and more of a conspiracy fact (and people tend to find "theories" to be a lot more fun).
The philosophical subtext around credibility is certainly interesting!