To turn your question around, how do some people KNOW that the truth they know is the ABSOLUTE [Complete, Total] truth?
And, how is it possible to PROVE this?
[Direct experience of something could "prove" it to yourself but even then your experience of anything is mediated through your senses and faculties, which could be limited; even if we have sixth, seventh and seven thousandth senses, there may be experiences that are out of range of even these senses, just as a dog whistle is out of our hearing range. Also anyone's experience of anything is filtered through their personal psychocultural profile; people can only experience anything subjectively.]
This whole subject falls into the realm of Things That Make Your Head Hurt (in a good way). On a more so-called down-to-earth level, though, I must admit that there are certainly things that I just absolutely KNOW beyond a doubt. It is wrong to rape. It is wrong to rob from the poor. Black people are not, never have been and never will be inferior to anyone. 2 million plus people are incarcerated in the U.S.A. Pollution is drastically and negatively affecting our ecosystem. etc.