Let's say you're an average person, of average intelligence, average education, with an average job, and you've run across several news articles.
One says that an asteroid has just been detected that will hit the earth in 2015. Another says that taking vitamin B3 daily can improve your cholesterol levels. A third says that increasing defense spending will help balance the budget. Another says that evidence of extraterrestrial life has been found in an Antarctic meteorite. A fifth one says that the Gospel of Mark has been dated as having been written between 40 and 50 CE. And finally, a story that people who prayed to a statue of Krishna have been cured of cancer and blindness.
How do you, as a lay person with a full-time day job, determine which ones to believe, and which ones to disregard?
I don't have a good answer, by the way. I'm hoping you can suggest something in the comments.
All such articles are trying to "sell" you an idea, in a broad, general sense. Sometimes the selling is literal, as when a company tries to convince you that you're a pathetic malodorous loser who'll never be accepted by the in-crowd or find true love unless you buy their product. Other times, it's metaphorical: "I want you to know this, because..." well, that's the question, isn't it? "Because we'll all benefit if people who will implement these ideas get elected." "Because I'll make a ton of money if you help elect people who'll implement these ideas." "Because I care about you and your health." "Because this will help save your soul from eternal damnation." "Because this idea, while bland, is true, and I think it's better if we know the truth."
It would be great if there were a single source to which one could turn to to get the truth, or if news articles came with a little checkmark, the way Twitter shows that "neilhimself" is the famous Neil Gaiman, while "NeilGaiman" is someone else. Unfortunately, that's not the case. The problem is that true ideas and false ideas can look an awful lot like each other.
But it occurs to me that nature has come up with a solution to this problem. In sexual species, males often try to communicate that "you should mate with me; I'll provide our offspring with plenty of food, and they'll be resistant to parasites and predation." In such cases, it's often advantageous to lie: a male who convinces a female he's in it for the long haul can impregnate her, then ditch her to impregnate someone else. Preferably while some other male sucker gets stuck caring for the liar's offspring.
So what's a female to do? How does she figure out who's serious about helping to feed the kids, and who's just trying to get inside her cloaca? One solution is known as costly signaling. "Signaling" refers to the "I've got great genes" message, above. The "costly" part means that the signal should be sent in a way that's difficult or expensive (in time, effort, ability, etc.) to fake. The usual example is that of the peacock, who demonstrates his worth by the fact that he's managed to survive despite having a huge, flashy tail that prevents him from flying, and hinders escape from predators. If he's managed to overcome such a handicap, he must have superior genes indeed.
The idea of costly signaling is more general than that: it basically means that the signaler has to invest enough effort or resources into the communication to be taken seriously, that cheating isn't worth it.
(As an aside, I can think of a few possible instances in human society: an engagement ring sends the message that "I'm willing to spend a pile of money on a small rock; so I'm in this for the long haul, not just for a quick fling". Taking a prospective client to dinner or to a ball game says "We don't do this for just anyone; but we're willing to do what it takes to get your business." And an Italian sports car and designer clothes say "I have so much money that I can afford to waste it on an expensive logo. Of course I'll be able to feed our family and send our kids to college.")
So getting back to my original point, it might be possible to identify costly signals to distinguish trustworth news sources from untrustworthy ones.
For instance, was the article published by a major news outlet, or by some local paper you've never heard of? In principle, the greater the reputation of the publication, the more editors and fact-checkers it has had to pass through to get published. Unfortunately, given the state of American journalism, this may not be as safe an assumption as one might hope.
A related criterion might be: do they have a fancy web site, or does it look like it was slapped together by someone's kid in the 1990s? Unfortunately, this doesn't work at all, since organizations like Americans for Prosperity, BP, and Answers in Genesis can easily afford good web designers.
Do the authors have letters after their name? An article on medicine written by an MD, or an article on science written by a Ph.D. is probably more trustworthy than one written by a beat reporter. The time and effort required to go through grad school or med school to obtain those letters should weed out the fakers.
Of course, the competence has to be in a relevant field: I tend to trust what Paul Krugman writes about the economy, because he has a degree and a Nobel prize in economics, but not if he writes about, say, medicine or geology.
And, of course, it's very easy to just say that one has a Ph.D., or to buy a degree from a diploma mill, without putting in the effort to learn a subject well enough to speak authoritatively about it. To combat this, there accreditation institutes that investigate schools and give their stamp of approval to the ones that require students to learn something before graduating. Of course, now that a lot of people have learned to ask "is your degree from an accredited school?", there are accreditation mills, which will accredit any diploma mill for a fee.
Has the author published any peer-reviewed research? Peer review is intended as a filter to make sure that research journals don't publish any old garbage. This criterion is probably pretty good, though not flawless. For one thing, it usually requires effort on the reader's part to seek out the author's publication record. For another, various creationist organizations publish cargo-cult "peer-reviewed" journals where articles are reviewed by a panel of fellow creationist before publication.
Trusted endorsements: this might be called the poor man's peer review. When Phil Plait, an astromer, writes a blog post that links to a post on astronomy, that's a good sign. It means that the article on the other end of the link hasn't raised Phil's baloney-meter. That tends to make me trust the article more, because Phil would notice errors that I wouldn't.
Does the site link to contrary views? In its heyday in the 1990s, one notable difference between the pro-evolution site talkorigins.org and anti-evolution sites was that talkorigins.org usually linked to the creationist sources they were discussing, and to creationist rebuttals of their articles. To me, this said "we're going to make it easy for you to read the other side's rebuttal, because we're confident that the facts are on our side, and even if you read both sides, you'll agree with us."
Any others? Ideally, the sort of costly signal should be something hard for the writer to produce, and easy for the reader to verify, without requiring too much effort (because we want to dismiss bogus claims quickly) and without requiring special knowledge. And if the criterion fits on a bumper sticker, so much the better.