Before defending a plan, identify the evidence that would make you choose differently. If no possible answer could affect the choice, the conversation may be seeking approval rather than information.
Write the assumption doing the most work. Ask a question that could test it. Keep the wording open enough that a person can disagree without having to fight the conclusion built into the question.
Ask where the consequence appears
In a fictional template project, the assumption is that supervisors need more reporting fields. Ask which decision currently fails because a particular fact is missing. The answer may reveal that the information already exists but reaches the wrong person too late.
Consider the source of the answer and the conditions under which it holds. One person's experience can reveal an important issue without establishing a universal pattern.
Record what the answer changes in the plan. If you retain the original choice, explain the trade-off honestly. A useful question creates room for a better decision; it also creates a responsibility to notice when the evidence points away from the answer you initially preferred.