You can't show someone is right, except in limited cases - all you can do is sometimes show that someone is wrong if you are able to show how their story is incoherent in its own terms.
Even then, if you correctly point out the inconsistencies that show a story cannot be true (as truth is consistent) if this person is acting in bad faith they can refuse to acknowledge your argument. This is the most common result - people do not want to be corrected and become aligned with truth - they want to be right which is more comfortable for their ego.
The deeper answer is that if you are aligned with truth, you do not care about winning an argument. In fact, losing the argument and disabusing yourself of a false understanding should be gratefully accepted as a blessing.
Ultimately, the 'golden source' is what each individual has personally experienced.
This is an epistemological question - how do we know what we know. Unfortunately, most of don't even know what 'knowing' means - we think that if others have told us they know that we know too, we think if we have seen something on a screen we know it. But the map is not the terrain. Anecdotal experience is king.
Knowing is only that which we have personally verified.
If you've got an "opponent", you've already lost. There is nothing in the world that can force them to change their mind. Even if they're right, they're not out to help you discover that. They're out to change your mind.
Instead, seek out situations where you're not opponents, but partners in seeking the best way for both of you to live together. It's not about seeking "truth", which is something you should both be able to agree on easily. If not, one of you is arguing dishonestly. It doesn't matter which, because the argument isn't going to help figure that out. You have to go find another way.
The world is full of differing opinions, and there's no real way to resolve them. We all have incomplete information. Even with identical information, and goodwill to admit it, you can still reach different conclusions. What we need is not to "defeat an opponent", but to seek ways in which both of you can live with the situation. Which is plenty hard enough even when you're not actively considering each other "opponents".
You'll still have to grapple with the fact that sometimes the things you know are wrong. That's incredibly difficult to get around. It's the essence of science, and of any critical thought. You gain experience through learning as much as you can -- and not with people who have made it important to their egos to change your mind.
So my advice: avoid the situation you're asking about entirely. If you're wrong, you'll find out via somebody more helpful, who is capable of understanding which things you're right about and how your picture of the world is misleading. Plenty of such people exist, but they're not the ones making the most noise.