How do you know if something is true?
There is an entire discipline within philosophy called "epistemology" which is dedicated to the problem of understanding how we come to and evaluate knowledge. First, it is important to understand that different types of knowledge must be evaluated differently.
Take the statement "The distance between San Francisco and New York is 3,000 miles". One could evaluate this statement by measuring the distance oneself or relying on the expert consensus one might find in atlases, online distance calculators, etc. In this case, its truth depends on correspondence to an external state of affairs.
The statement "a bachelor is an unmarried man" is true in a different fashion. One doesn't need to investigate anything in the world to determine this, but "unmarried man" is part of our definition of the term "bachelor."
Statements like "torture and murder are morally wrong" depend on belief systems, and can only be evaluated within the context of those systems. Other statements such as "I prefer pizza to hamburgers" are reports of internal states -- de gustibus non est disputandum.