Discerning Good Source Material

One thing that's important for anyone who relies, to any degree, on sources outside themselves for spirituality - or anything else - is being able to judge a good source from a bad one. So today I want to just run down a quick list of ways to vett sources of any type to decide how much weight you should give to something. Even if a source isn't perfect it may have value - or it may be immediately tossed out. It depends on how it measures up.

1. What sources does this source use? - One of the first things I do with any new source, be it written, video, in person, or what-have-you, is to try to look at what sources that source is using. Are they talking purely from personal gnosis? Are they using academic texts? Are they using other authors based in personal gnosis? Are they using well known and respected sources? Are they referencing conspiracy theorists or known white supremacists? Do they have no sources at all that they admit to? All of these things need to be taken into account. Something that's entirely personal gnoses isn't necessarily bad but needs to be understood in that context, while something from a deeply flawed or problematic source will be eliminated. 
2. Never once the Wikipedia - Okay this is  bit ranty right here, but as soon as I see wikipedia listed as a source for anything I'm done with that source. There's a very good reason that wikipedia can't be used in college, university, or even high school classes: its notoriously unreliable and oddly biased. Anyone can and does edit wikipedia and while its true that wikipedia cites sources and includes references pretty much any print or online source can be used and there is no quality control. Let me repeat; there is no quality control. The entry on Baobhan Sithe was sourced mainly from modern vampire guides, themselves largely repeating modern urban legends, and from RPG guidebooks. No really. The entry on Finnbheara contained an assertion straight from a fiction novel (I removed it, because remember anyone can edit wikipedia). Please don't trust anything on wikipedia or any article using wiki as a source. Just don't. 
3. What is the author's bias? - Every author or teacher has biases, that's just human nature. Figuring out what to think of a source means understanding what that source's biases are and how that's affecting the material. A bias doesn't mean you can't use a source but that you have to be aware of the way the author's opinions influence their work. To use myself as an example - I am unashamedly nativist in my views of Irish mythology and folklore. Nativism is a bias that means I will always tend to see material as having some native Irish influence or value in it; anti-Nativist in contrast means that the author tends to always see foreign influences in any historic Irish material or mythology. Neither is necessarily provably correct or incorrect but both strongly influence a person's views. Authors can have all kinds of different biases and its helpful to just be aware of them or at least that bias is a possibility. Even a book that is aimed at sharing facts will still be influenced by the author's personal opinions and views. Be aware that bias is a thing and that it matters. 
4. Date - Another thing to consider is how old the source is, particularly for books and articles. Scholarship is always changing and evolving and when I was in school we were strongly encouraged to use material that had been written within the last 10 years and preferably within 5. That was in the field of psychology of course and in more casual study you don't need to be as strict with this but the core idea is the same, that older books tend to have ideas and theories that are more outdated. This doesn't mean the whole work is useless, just that it needs to be kept in context. For example I love the Fairy Faith in Celtic Countries and find it valuable for the anecdotal material - yet the material written by the author themself which waxes eloquent about fairies as Bronze age pygmy survivals in iron age Britain is clearly not only out dated but thoroughly disproven by actual archaeology and anthropology.  
5. Perspective - what perspective is this source speaking from? Is it being written by a member of the community? A believer? A non-believer? An outsider? A scholar? A laymen? Like bias the source's perspective on the material also needs to be understood in the context of its value, because someone who is part of a community writing about that community has a very different perspective than an outsider, and a scholar has a very different perspective than a laymen. Each voice can have value in a discussion, but we shouldn't forget where each one is speaking from. 
6. Non-fiction or fiction? - this may seem like an odd one, but I see a lot of blurred lines between these two in some cases, possibly because older folklore is often treated as fiction and so modern fiction is given the same weight as folklore. It's worth keeping in mind though that folklore represents stories that people believed to be true (as opposed to fairy tales, which are something else) while modern fiction is the work of imagination. How fine or thick a line there is between those two will be a matter of opinion, but it should at least be considered when weighing the value of a source, whether it was written as fiction or not.