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Ask each side to give a probability. If both sides have a similar estimate, then you move on to the next logic step(s). Don't bother debating things you already agree on. We don't have to dissect everything.

If it varies too much, then explore the reasons for the different estimates. Often times it comes down to different past experience, which is obviously going to vary per individual. (Ex: "when users see X on UI screens, they usually do Y, in my experience".) But at least one can narrow down recommended decision logic to observed patterns of history. Narrowing down the point of disagreement is often still helpful even if both sides still disagree.