Welcome to the modern condition; this will only get worse for the foreseeable future.
The easiest heuristic is always to first ask qui bono, and then heavily discount information whose content if true would advance the interests of the information supplier.
Thus: if Pepsi runs an ad on TV saying Pepsi is the schizz and Coke is just fizz, you discount that information heavily b/c Pepsi's just talking its book.
There's a loose analogy to information-theory proper here: the information content of a message is proportional to its unpredictability; if for example you could reliably reproduce an N-bit message once you had the first k < N bits you'd say the message only has k bits of information, not N.
Thus if knowing who is speaking lets you know what they'll say you should just ignore them; if you ever want to find out what they said you just have to remember who they were and reconstruct what they were saying, so you can defer considering it until you have a reason to care.
This looks like ad hominem but isn't; it's just avoiding low-information-gain use of your time.
The other thing to do is to back away from the twin notions that:
(1) holding true beliefs is in and of itself as valuable as you think it is
(2) most participants in a debate (a) attach the same importance to holding true beliefs that you do and (b) would switch to a different belief if the evidence supported it
(1) is probably the hardest to let go, but it's something you have to let go (holding true beliefs when such beliefs are inputs to an important decision is important; otherwise not so much).
(2) is more important.
A good general rule of thumb is that most people like to think of their lives as having had some kind of meaning or purpose, etc.; just living, enjoying what life brings you, and peacefully shuffling off the mortal coil isn't a satisfying life for many people (probably b/c our brains are hardwired for a difficult life in the jungle and get bored in the absence of real threats to overcome).
In places where life is nasty, brutish, and short survival itself is enough of a challenge that the struggle for simple survival is itself enough to cast your life into a narrative where you're a hero of some sort; in modern societies the challenges of life aren't generally enough on their own to provide a convincing narrative of moral heft and so other struggles have to be invented to fill in the meaning.
A first consequence of this impulse is firstly that people tend to assign excessive moral heft to the things they're good at:
- someone who has eg bootstrapped themselves out of a bad situation by virtue of wise decisions and self-restraint will assign a high moral value to wise decisionmaking and self-restraint, so that their life's story has a heroic bent to it above-and-beyond simple utility maximization
- someone who has strong aesthetic or artistic preferences or beliefs will assign a high moral value to aesthetics and artistic purity so that the decisions they have made (not "selling out", etc.) take on a heroic bent above-and-beyond simple utility maximization
- and of course someone who believes themselves to be good at arriving at true beliefs will assign an irrationally high importance to holding true beliefs (if you doubt this just count the # of times pundits or would-be "thinkers" write sentences like "I used to believe X but then I learned about Y and after some struggle I now believe Z instead X")
...etc. and a second consequence of this impulse arises from how it interacts with social life; people are at heart social beings and thus it is very easy to add additional axes of meaning to one's narrative by banding together into a faction as part of a larger struggle against other faction(s).
These affiliation-derived meanings range from sports teams to political labels to "sides" in an argument or public debate.
These two dynamics intersect in areas of contention in predictable ways:
- on each side of an argument you'll have some nucleus (core members) who will be a mix of people who assign a huge important to true beliefs (and currently believe in the side they're on) and shills (representatives of those whose interests hinge on what way the debate gets resolved if it gets resolved)
- then thanks to the way that additional meaning can be added to people's lives the mechanism by which affiliation-derived meaning can be added just by taking a side means that many hangers-on pick sides based on the meaning it gives them to do so (or because they've already made a commitment to a faction in exchange for enhanced meaning and "in for a penny in for a pound" (eg: joining a church makes it easier for them to cast their life as partially a struggle for virtue and against sin; b/c of that prior commitment they also commit against evolution as in the church they joined they can't do both))
With the following consequences:
- the people who hold true belief important can be convinced by reasoning and argumentation, provided the argumentation is sufficiently, overwhelmingly powerful that it's impossible to see said argumentation without coming to agree with it as a rational being; most argumentation -- especially on debatable issues -- is not that strong, and thus not enough on its own merits to nudge someone on the "other side" to take the hit to their self-esteem (as a discerner of true beliefs) and switch
- the shills can't be convinced by anything; they're dishonest participants and will argue their case as long as they keep getting paid
- the affiliational-types aren't going to be convinced by reasoning; the get convinced only when it becomes clear that greater meaning-in-their-lives can be had by switching to the other side, which is more about reconciling conflicting affiliational claims than it is about winning a debate (eg: finding a away to reconcile their religious beliefs with the other side -- thereby letting them keep their religious affiliation and join your side -- is more important than convincing them your side is right). A huge downside of even trying to argue is that arguing with them increases the amount of meaning their participation in the struggle gives them -- as it starts to look and talk like a real struggle, with conflict and bad guys and so on -- best just to make your side seem like the more-meaningful side
Which en fin is why such issues are best ignored. Things will only get worse as technology improves: allowing more communication => more opinions transmitted to more places => more factions / debates (and thus less time to learn the intricacies of each of them even if you wanted to) => more opportunities to join factions and add meaning to life => more of the same only more so.
This will be especially true for historical stuff, since history is vastly underdetermined by the available evidence and the same set of historical facts can usually support dozens of reasonable interpretations; much of the apparent previous state of consensus about what happened historically is as much a function of the previously high barriers to entry in publishing stuff as it is a sign that the previous consensus opinion was correct.