I can't recall where I picked this up, probably from a Michael Shermer podcast, but I wrote it down as its simplicity appealed to me. 

When encountering a conspiracy theory, or those who traffic in them, simply ask the following:

1. What is the evidence for your claim?

2. What is the source of the evidence for this claim?

3. What is the reasoning that links your evidence back to the claim?

Expect to hear some stammering or see some tap dancing.

And always be mindful of the Hitchens quote: " What can be asserted without evidence can also be dismissed without evidence. "

Peter T.