Truth is subjective. That's why Socrates said: The only true wisdom is in knowing you know nothing.
Knowing nothing doesn't mean that you actually don't know anything. It means that you should not start the argument by assuming that your position is the ultimate truth and others are wrong. Keep talking, enhancing your position, seeking for the truth, and don't be afraid to say: I was wrong.
How can you tell that you've found the truth? The only thing that you will find is the best argument, so far. Because I must have my believes that I can change if there are stronger arguments. That's why I can keep changing them; my ideas, concepts, and believes. It can be a better argument for me and bad for others.