The same way you determine whether or not 2+2=4, or whether or not Suharto is a mass-murderer, or Henry Kissinger a vile criminal. Note that the fact that 2+2=4 is a simple problem does not mean that the manner in which you figure out it is true or not is any different from other, more complex problems. You have to do the hard and familiar work: investigate, sift evidence, debate, discover, compare, etc...
>The debate about what is true is far more contentious than you seem
>to assume (e.g., the Kosovo debate). ...
I neither said nor implied that debates over what is true would not be contentious, just that one should not do as Besencon said one should do: "refuse --- without discussion --- the description of reality that [the other side] proposes ... [o]nce you grant that their description has an 'element of truth' ... you're lost". I think this is foolish, misguided, and intellectually lazy. I think Doug's claim that "[t]he moment you accept that there's an element of truth to lesser evilism, you're lost" is just absurd. The claims that democrats are in general the "lesser evil" necessarily contain a germ of truth --- that's what makes them so powerful, as with all smart propaganda. We should recognize the germ of truth and destroy the lies that are gathered about it. If someone says "The Serbs are behaving in a rotten way, therefore we need to bomb them until they stop", I will readily, even enthusiastically grant the contention that the Serbs are doing rotten things, but I will disagree with the conclusions. I'll ask it again: "What are we supposed to do, *deny* that there are elements of truth and retain the Manichean Image"? Should I deny that the Serbs are doing the wrong thing if I wish to disagree with the conclusions reached? I think not, and further, I see no way of discussing *anything* of substance if we were to adopt Besancon's advice.
>to assume (e.g., the Kosovo debate). It is not a question of
>demonizing the enemy; it is a question of not allowing your enemies
>to frame the issues in a way that requires you to play their
>rhetorical games. ...
I explicitly said that one not accept how the debate is framed ("who says we must accept these arguments as framed", I asked), and I think that we should not accept how debate is framed if it is immoral, misinformed, or otherwise obnoxious. In fact I also asked why we accept the "lesser evil" framework for debate in the first place, since we don't vote for "Democrats", but for particular persons. I said we needed to drop this framework and work "on a case-by-case basis". I was explicitly addressing what was quoted by Doug, viz, you must not admit that the position of the "other side" contains an element of truth, or "you're lost".
I'm very well aware of the Miller-Lite style of political debate in this country: Tastes Great vs Less Filling, with no question that we should be drinking Miller Lite.