Just like lawyers who need evidence in order to prove a case, you need evidence to build a compelling business case. You have to prove together with the client that there really is a problem or opportunity, and clearly establish the criteria for success.
Use “how,” “what,” and “where” questions to gather evidence.
HOW specifically does this show up as a problem?
WHERE does this show up as a problem?
WHAT is going on in your company that proves this really is a problem?
HOW would you know that you were being successful?
WHAT measurements would prove success?
WHERE would indicators of success actually show up?