It's good that you classify this as for "beliefs of the form 'policy X will result in outcome Y')". That may be answerable. Much political discussion and dispute is more about relative preferences and goals than matters of fact. For example, gay marriage. Few people honestly dispute any matter of fact in the gay marriage debate, nor would many people's minds be changed by learning they were wrong on a matter of fact. It's an argument about values and preferences.
I've seen a number of disputes of fact on this matter. It's likely that these aren't usually true rejections of gay marriage, but I doubt all the people who express these concerns are straight-out lying. This article seems sincere for example.
People agree that gay marriage will increase the number of same-sex couples raising children. Is having a pair of opposite-sex caretakers crucial for a child's well-being?
Will allowing gay marriage increase the prevalence of open marriages?
Will allowing gay marriage increase STD prevalence?
So you're playing the credence game, and you’re getting a pretty good sense of which level of confidence to assign to your beliefs. Later, when you’re discussing politics, you wonder how you can calibrate your political beliefs as well (beliefs of the form "policy X will result in outcome Y"). Here there's no easy way to assess whether a belief is true or false, in contrast to the trivia questions in the credence game. Moreover, it’s very easy to become mindkilled by politics. What do you do?
In the credence game, you get direct feedback that allows you to learn about your internal proxies for credence, i.e., emotional and heuristic cues about how much to trust yourself. With political beliefs, however, there is no such feedback. One workaround would be to assign high confidence only to beliefs for which you have read n academic papers on the subject. For example, only assign 90% confidence if you've read ten academic papers.
To account for mindkilling, use a second criterion: assign high confidence only to beliefs for which you are ideologically Turing-capable (i.e., able to pass an ideological Turing test). As a proxy for an actual ideological Turing test, you should be able to accurately restate your opponent’s position, or be able to state the strongest counterargument to your position.
In sum, to calibrate your political beliefs, only assign high confidence to beliefs which satisfy extremely demanding epistemic standards.