How do we know the truth about something?

We can know the truth about something by investigating or by proving that that is the true about something. For example how do we know that 1+1=2? well we will know this if we count 1+1 with our fingers and then we will find out that 1 plus 1 is 2. According Socrates each person is his or her own measure or standard of what is true or false. what seems true to me is true. What see true to you, though it contradicts what i believe, is true relative to you. in the same way, your pain is rue for you, and my pain is tryly painful for me. So everyone have or may have their own defenition about true or what can be or not be true.