Ways, to check sources creditability, is to in my opinion do a Google search. Even though it sounds simplistic, my old debate teacher in high school would put certain phrases in Google. And it was his way of checking for plagiarism. Which in many cases, I can see how it would take the hard strain of checking for creditability out.
The second way would be to thoroughly read, and check if the sort of writing style that is given. Matches the writing style of the person, the best example I can think of is. Would it be something they would say normally? If it is, then I would agree to the fact, that the person wrote it. If it does not use terminology, they would use, then the possibility of either plagiarism. Or someone else wrote it.