This isn't quite right - for example, the more I search and find only bad arguments against cryonics, the more evidence I have that the good arguments just aren't out there.
If all you did was argue with stupid people you would become erroneously self-confident. Also, two people who argued and didn't convert would both walk away feeling better about their own positions. Something seems wrong here. What am I missing? Doesn't this only make sense of there was some sort of weight attached to the argument your opponent used that was detached during the argument?
Apply Bayes theorem. P(you don't find a good argument among stupid people | there is a good argument) is high. P(you don't find a good argument when you've made a true effort to scour high and low | there is one) is lower. Obviously the existence or otherwise of a good argument is only indirect information about the truth, but it still helps.
That it seems there are no two people who cannot disagree without both having the strong feeling that their argument was clearly the stronger must of course be borne in mind when weighing this evidence, but it's evidence all the same.
Followup to: The Most Important Thing You Learned
What's the most frequently useful thing you've learned on OB - not the most memorable or most valuable, but the thing you use most often? What influences your behavior, factors in more than one decision? Please give a concrete example if you can. This isn't limited to archetypally "mundane" activities: if your daily life involves difficult research or arguing with philosophers, go ahead and describe that too.