Method to assess text credibility
I don't think there's anything close to doing this:


It would be very hard to even define the task objectively, as different humans wouldn't agree about what is credible or not.
It would require a complex system to represent reliable background knowledge... and again people wouldn't agree what should be considered "reliable" or not.
Generally the state of the art in NLP is still far from solving tasks related to Natural Language Understanding in a satisfying way. Juding the credibility of a text requires not only a real understanding of the text but also an ability to reason at a higher level. It's not clear whether this level of AI can ever be reached. 


If you find a package which pretends to achieve this task, try to apply it to its own documentation because it's not credible ;)