That depends on what it's a fact about. If it's a fact about the physical world, I use my senses. If it's about mathematics, I use mathematical methods (e.g. proofs). If it's a moral fact, I reason about whether it's something that one should do.
How do you know if your reasoning is correct and someone else's (who disagrees with you) isn't?
As per a recent comment this thread is meant to voice contrarian opinions, that is anything this community tends not to agree with. Thus I ask you to post your contrarian views and upvote anything you do not agree with based on personal beliefs. Spam and trolling still needs to be downvoted.