I may disagree with a NYTimes writer, but I know what his/her background is, I know, in general, where to find the facts in the article, and I know that some vetting has been done of these facts. I also understand the paper's corporate bias.
Individual citizens, however, vary widely in their use of fact and vetting of it. I really have no place to start from and do not always understand their biases. Sometimes it's easy: an individual citizen can write about space aliens or fluoride in the water, for example, and I can rule them out. However, individual citizens can also write opinion pieces with little vetted fact supporting them or they can take facts from sources which distort them. Or, sometimes, individual citizens can be 100% correct and very credible.
In order to assess credibility, you have to look at the strategies a given individual employs. When an individual citizen cites a "witch hunt" piece of signage without a link and uses it as an exemplar of current rhetorical tone without providing evidence, credibility takes a hit.
To get an idea of a credible individual citizen post, take a look at this one:
http://www.democraticunderground.com/?com=view_post&forum=1002&pid=6295819
The poster builds an argument with credible sources. She is not simply venting her opinion and using an inflammatory historical poster to up the emotional ante. She has a definite perspective, but the facts to back it up.
I trust marym625 because I can trace her facts and because she builds her case well.