In other words, you consider what they say to be truthful and accurate within the scope of their knowledge. You can trust that they are not deliberately lying, manufacturing information, or seeking to mislead.
When a person is a known "spinner" (a polite word for "liar"), then a far more suspicious attitude has to be taken to any assertion he makes. Colloquially, "if he said the sun rose in the east, I would go outside and check."
It is what I said, if something is beyond your ability to judge on its merits, you have to use trust.
It does not have to be disparaging, I had to focus on personal trustworthiness of my dentist and car mechanic, since I am not able to form judgment in their areas of expertise. It was a painful trial and error process before I found the good ones.
But this is political forum where we are supposed to be able to debate on our own. Otherwise we might lose chance to remain quiet :)