Look for people who can state your own position as well (or better) than you can, and yet still disagree with your conclusion. They may be aware of additional information that you are not yet aware of.
In addition, if someone who knows more than you about a subject in which you disagree, also has views about several other areas that you do know lots about, and their arguments in those other areas are generally constructive and well balanced, pay close attention to them.
None of us are perfect reasoners. None of us have unlimited information. Sometimes other people are more correct than we are. This is an obvious thing we all know but may not practice .
Below are some concrete questions you can think about that come at this problem from a very specific path of trust (There are a lot of other ways to come at this). Note, that when I say "trust" I AM NOT NOT NOT saying that you should allow other people's viewpoints to completely supercede your own. The trust I am implying is more along the lines of something like "this would cause me to pause and consider."
Who do I trust to be a solid representation of an alternative viewpoint?
The most frequent arguments we hear for viewpoints different from are own are often not the strongest arguments. Who do you trust to present well-reasoned, not mindkilled arguments for a differing viewpoint, while not trying to persuade you with ideological rhetoric.
If I am in the throes of emotion, who could tell me that I wasn't behaving "rationally", AND (ideally) that information would actively cause me to change my behavior?
Particularly if you know you often behave in ways you don't endorse when you are angry, or starry-eyed, or perhaps under the influence of illicit substances... Is there someone who could say "You need to stop what you're doing right now." For me, there are occasional times that I am angry-and-irrational enough that I explicitly seek out an outside view as a check on my reasoning and actions. Any Friend I Trust is going to be a better judge of my actions at that time than I myself would be. Later, when I'm calmer, I can revisit and make sure I still endorse those conclusions.
Who would I most trust with end-of-life care?
This is a situation it is good to prepare for in advance because you may be completely unable to effect the decision by the time it comes up. So in this case you actually ARE putting important personal decisions completely in someone else's hands. My person is actually a former partner that I haven't spoken to in over a year. But we've had explicit discussions about our preferences (which are very similar), and I trust him to not be swayed by things like "What will other people think?" or "I need to prove my love by not letting go."
Next, do this in reverse. Think of some specific friends, and for each friend think about what they, specifically, as individuals, have to offer.
What is the sort of thing Alice In Particular is likely to get more correct than you? How much are you willing to pause, reflect, and potentially act on their input? Below are some axes of trust you can think about with each individual?
(It could also be interesting to do this with some people you don't particularly like or trust. Is there something you would still trust them with? Maybe they have some domain knowledge and if they said "Your math is wrong" you would update enough to go back and check your work or ask for clarification.)
Axes of trust:
Do they have domain expertise that you don't have? Are they generally more intelligent or rational? Do they have a different but useful worldview? Do they have past experience you don't have access to (e.g. coming from another culture)? Do their incentives and goals align with yours? Do they care a lot about you? Do they have an accurate model of you and your options/ limitations?
If anyone has more axes or questions of trust to consider, I would love to hear them.