How do YOU? I've been around over 8 decades and have done a lot of thinking and Life.... How long have you been around? Just curious I guess.
I'm older than Matt Ross, but younger than Catherine Tate.
And I will answer your question in good faith, which you have not done. I decide what to believe by basing my worldview on reliable, repeatable evidence. The best tool to gather that evidence is science. In science, we test our hypotheses. We document our methods. We record our results. We review other peoples' results. Most importantly, we change our minds when contradictory evidence emerges.
When someone makes a claim like "grape seed oil prevents the common cold", we don't just take them at their word. We perform a study. We gather a large number of people and divide them into three equal groups. One group is instructed to take grape seed oil for six months, one group is instructed to take an inert sugar pill for six months, and the last group isn't instructed to take anything. Then we record the frequency with which each group reports cold symptoms. If the grape seed oil group reports fewer instances of symptoms than both of the other groups, then we conclude that the grape seed oil may have been effective.
What I've just described is called a placebo-controlled trial, and it is one of the most useful and reliable tools for determining truth from fiction in the medical field.
That's how I determine what to believe: I look at what people who are knowledgeable in the field are saying. I check to see whether other people who are also knowledgeable are saying the same thing. I trust in the expertise of people who have studied a field academically for years or decades, over those who make YouTube videos and think that "research" means looking it up on Google.
Now, are you going to do me the courtesy of answering my question, as I have done for yours? Or are you just going to dismiss it again?