From this website I was introduced to one of the best explanations of how science works. Thanks to Brian for the link.
Science is an area of study where the means justify the ends. You shouldn’t start with a conclusion that you like, and then form an argument around it (that’s why I’ve never liked literary analysis). The scientifically correct procedure is to follow approved and tested methods, and see what conclusion comes out of that.
Apply this idea to your assessment of credibility. In addition to investigating the expertise and relevant education of the source, ask yourself whether they’re following the scientifically accurate method of gathering evidence first, or if they’ve started by choosing a politically acceptable conclusion to stick to no matter what.
Does the National Academy of Sciences study science by starting with hard data and evidence, or by starting with a conclusion that they like?
What about the Heartland Institute?