"t the very least there should be some transparency about how the media decide whether the source is credible."

I recommend you read All the President's Men.  It details how Woodward and Bernstein were required to find at least two independent sources for every story.  They had to identify those sources to their editor, Ben Bradlee, who had to be convinced those sources were credible.   No matter how juicy the information, it could not be turned into a story until that vetting process had been completed.

That is standard-practice journalism.  In practical terms, the sources reporters take information from are people they know and have dealt with before.  They know their sources are reliable.  People who feed reporters false information that cannot be verified do not get to be sources.

But, yes, news organizations should make clear how they vet sources and the information they provide.  People need to know how journalism works.