Push triangulation. Hit at least three sources - one has to be outside your ideological vacuum chamber. Any one of the three betrays dubiousness, spread out to at least two more sources BEFORE reposting, tweeting, etc.
Proposal: Ask straight out - "Doesn't sound right. Did you triangulate your sources?" #triangulate The responder should be able to give three URLs upon request within a reasonable amount of time. Otherwise the tweet/post comes down.