Kythia, if I can offer a suggestion that might help:
It's functionally impossible for anyone to know everything about everything, of course. So it becomes necessary for us to just trust certain things to be correct. Thing is, people often misunderstand... incredibly misunderstand... just what that "trust" means. It doesn't mean that if someone tells you something, you decide, "well, this person has never lied to me before, therefore what they say is likely true." In fact, that's not true at all. My brother has lied to me a bazillion times before - he used to make a game out of telling me crap when I was younger! - but if he told me that his kids want this or that game for their birthdays, then I'll trust him.
So what exactly is going on there? The key is that you decide on the truth of something you hear NOT just based on who it comes from, but on whether or not they SHOULD know about that fact. Take Carl Sagan for example: I don't know him at all as a friend, so I obviously I would trust my friends more than him if it were just a matter of familiarity. If it were a question about whether I should do this or that with my life and circumstances I would trust my friends far more than Carl Sagan; they know my life and circumstances, he doesn't. (Obviously I would trust myself still more, because I know my life and circumstances even better than my friends, and unlike, say, an addict of something, I have no reason to doubt my rationality.) But none of my friends are astrophysicists who taught at Cornell and helped put a man on the Moon, so if the question is about space or physics, Sagan's my man.
The same applies for the flat lemonade idea. Suppose a mother hears from a friend that flat lemonade is great for coughs. She may like the friend a lot, and may consider them a very trustworthy friend... but unless that friend is a doctor, it would be stupid to take medical advice from them. If you want to know how to treat a cough, you ask a doctor - or, to be more realistic, a doctor proxy, like a respected medical help line or a website. But how do you determine "respected"? It's simply taking another step back and doing the same process. You find someone or something else that should know about medicine, and see what their opinion on the site is. If you're really concerned, you should also check the criticisms of that site, and how well respected the source of those criticisms is. The more important it is to you that a certain fact is true, the more work you should put into digging down into the foundations of its truth.
You don't need to learn medicine, biology or pharmacology to weigh the truthfulness of the flat lemonade claim. You just need to know that doctors, biologists or pharmacologists are more likely to know whether its true or not than some friend.
So I disagree with what you say about parents teaching their kids religion just because they believe it, no matter how strongly they believe it. If they really care about their kids, and about their kids choosing the truth - and if they really believe that their beliefs are true - they should challenge the kid to find the truth on their own, while providing them with the right critical thinking skills to do so. They should teach the kid, "do not believe what we believe just because we're you're parents, or just because someone else in authority tells you its true - check not only the reliability and honesty of your source, but also whether they should be expert enough on the topic to give you advice." Not, "just trust us because we're you're parents and wouldn't lie to you."
So basically my suggestion is: don't try to untangle the facts of a claim unless you have the expertise to do so yourself. And don't simply trust someone who has your best interests at heart. Instead, try to figure out who should have the expertise to know the truthfulness of the claim (and, obviously, no reason to lie about it), and ask them. And check the opposite opinion, and their credentials as well.