I started wondering about this after reading this interesting essay about how Joseph Stalin isn't exactly how he is popularly made out to be; that, in fact, Stalin tried to democratize Russia and Kruschev distorted his legacy with de-Stalinization. Here is the essay for anyone interested: http://clogic.eserver.org/2005/furr.html
I spend some time around /r/Communism and /r/Socialism a bit and those folks like to toss around terms like "Western propaganda." What makes historical accounts trustworthy or distorted propaganda? Is history ultimately bastardized by relying on secondary sources? Why is it that if I were to cite Prof. Noam Chomsky or Glenn Beck then members of academia will roll their eyes? How can I know who is and is not okay to cite? And finally, how can we be sure that we really know what we know and that our knowledge is not distorted in the interest of political or economic elites?
There's no straight answer to this, just ways to figure it out.
Source reliability can only really be determined from context, and there are many factors which go into figuring it out.
A secondary source used in an academic discussion should be peer reviewed, either via an academic journal or an academic press. This means that certain works may be acceptable while others from the same source might not. If you cite Chomsky's linguistics work, for example, you won't get eye rolls.
You need to know how a certain argument fits into the broader discussion going on in the field. Who is the author reacting to, what tradition are they coming out of, and what are the fundamental assumptions that they hold? How in line are they with the main stream of thought on the subject, and if they deviate, how is that deviation supported? This is one of the reasons why doctoral candidates in the US spend two years preparing for massive comprehensive exams on their general fields of interest before they're allowed to begin working on their dissertation.
What sources do they used to back themselves up? Do they show a grasp on the field as described above in their footnotes? Are there any important sources that they've neglected? Are they using the latest editions? Are they current on major arguments?
That said, I find it absolutely infuriating to have a source rejected as "propaganda" or "liberal bias." If you're going to say that a source shouldn't count, you need to say why, and that "why" can't be "it disagrees with what I think."