the question seems easy because in daily life you are convinced of hundreds of things. you have an opinion on (almost) everything. well, is that really true? how do you know that the issues/topics that you are convinced to be true are really true? how do you know that your political opinions are true and don´t just support your prejudices? in answering this question you will find tons of arguments that proof your view/your truth. does that mean that your truth is true?
in my experience this leads us to the question if we want to be honest to ourselves. you might respond with a big YES!
another question: what does truth(or what you think is true) serve you? what do you need it for? to illustrate: when listening eg to political discussions sooner or later (depending on your personal political stance) you might either get bored or agitated or...... bored because these discussion follow a certain repeating pattern: every party thinks of itself to possess the truth instead of seeing its own perspective as a part of the whole based on (unconscious) assumptions about reality. assumptions on the other hand are attempts to find meaning and understanding in a world that otherwise would be hard to be explained(which is an assumption itself!). the problem with assumption: one doesn´t see them as means to understand but as convictions. that´s the moment you get attached and become a believer. the moment you become a believer you ar convinced that you own "the truth". extremists are firm believers.
what would happen if you place a question mark behind every belief?
how does that feel? maybe you would loose your sense of structure and safty in a world that seems to get more complex every day. nothing to grasp! nothing to hold on to?
at the same time this is a chance to see that your beliefs are just illusions to help you get a safe passage through the journey of life. pointers at best. actually this can be the moment in your life where you start your real journey: the journey to yourself and the deeper issues that you will meet on this path!
to answer the question "How do you know that you know?": in a philosophical and spiritual sense we might conclude that we don´t know at all. this recognition can point us to the "truth" that in a pragmatic sense (to cope with daily life issues) we "should" see (our!) knowledge as temporary and relative, as part of a bigger picture and that it makes sense to see others as pointers to expand our view of life!
another angle of view: to get an understanding of your individual truth: silence is a great space to ponder every question you have. answers will arise. maybe just not at a time you want them to arise!
PS: you might get agitated in political discussion because you share the same opinion of one of the parties and reject opinions of other participants!
what would happen if you start opening up towards views that are contrary to yours, get curious about the assumptions behind arguments(even your own!). what would change?