Actually, you (you personally, not some general "you") often don't notice when you lose on this forum, because people give up on you and disengage.

But how does anybody know who was ultimately right?

Outside view!

You're a smart person on Less Wrong. So are your opponents. My prior for you being on the right side of the debate is < 50%, by symmetry. (I assign a nontrivial probability that both participants are wrong.)

I can know that something is happening statistically without being able to point to a single definite instance of it.