You don't - that is life. You can make judgement, opinions and so on, and then you can test them by asking people for their thoughts, looking for proof or disproof , trying things out (carefully if possible), gather data etc.

This takes a lot of time and energy so you only do it for the important or startling things, the rest of the time you kind of run on autopilot using your knowledge, experience, education, senses, intuition - you get the picture I am sure.

Most people get much partially or completely wrong and you change views and thereby add to your experience and knowledge. Stubborn folk are rather slower to change than is quite wise, but they have their reasons. It is no big deal to get things wrong - you are just one step dafter than the next person. I like Winston Churchill's comment: [I have] had to eat my own words many times, and I have found it a very nourishing diet.