I appreciate I am not the intended recipient of this question, but let me be unpolite and answer it anyway.
I think the credibility of a news source is a bit like the correctness of a scientific theory (I said 'a bit', I'll hope you'll cut me some slack). Theories cannot be proven right; they can be disproved though, and that's the best you can do.
Likewise, it's pretty difficult to say a news source is trustworthy; but it's very easy to see if one is not.
In particular, it is sufficient to compare what the news source says with official documents, with prime sources, which they usually refer to. I'll give you an example.
A couple of years ago, Italy was gripped by a debate about the ESM (European Stability Mechanism); in particular, discussions raged about whether the reform would still imply that any ESM intervention would entail conditionality on the country that needed it.
Lots of mainstream sources claimed it was not the case, arguing their case with a very lose interpretation of statements offered by various officials during press conferences. But anyone who looked up and read the official documents, or simply the European Treaty of the Functioning of the European Union, could see, in black and white, on paper, that conditionality had not (and could not) be removed.
A simple check was enough to reveal all those mainstream sources as false, and not worthy of trust on the subject.
You repeat this process, and with time you build a list of non-trustworthy sources, which you are probably not gong to waste your time on; once a liar, always a liar is a good principle, already stated back in Roman times.
Alternative sources are mostly crap; but a lot of them might contain links to original sources, or documents, and this makes them valuable, because links to documents of this type are usually very hard to come by in mainstream media. This is precious, and allows the interested reader to access prima-facie, non-filtered information. Which is why even crap alternative sources are worth posting.
This is all my 2c of course.
IPB