Ambition and fantasy look the same in a business case. Both are bold, both are exciting, and both have a plan that works if nothing goes wrong. The difference does not live in the vision. It lives in the constraints, and whether anyone has been honest about them.
An ambitious programme is one where the goal is hard but the path is real. The constraints are known, they are uncomfortable, and someone has done the work to show how time, money, scope and capability can actually meet. An undeliverable one is where at least one of those constraints has been wished away, usually quietly, usually to keep the case approvable.
The fastest test I know is to attack the dependencies, not the goal. Ask what has to be true for this to work, then ask who has confirmed each of those things and when. A feasible programme has answers that survive a second question. An infeasible one has a chain of assumptions where each link is "the other team will be ready," and no one has checked with the other team.
Capability is the constraint people lie to themselves about most. A plan can be perfectly logical and still be undeliverable because the organisation has never done anything at this scale, and the timeline assumes it suddenly can. Money and dates get scrutinised. The honest question of "can we actually do this, with the people we actually have" gets skipped.
I have learned to watch how a team responds when I pressure-test the case. If hard questions are welcomed and answered with evidence, that is usually a real plan and a team that has already worried about the right things. If hard questions are treated as a lack of belief, that is the tell. Defensiveness is where infeasibility hides.
For a board, the value of asking this early is enormous. The most expensive programmes are not the ones that fail at the end. They are the ones that were never feasible at the start, and absorbed two years of investment before anyone was willing to say so.