The drill
Five questions, run on the one thing you built
Run against a single interaction, after it works, to find where it does not.
The five questions are ordered by how easy they are to skip. The first two get answered in the planning; the last three only show up once something real is on a screen.
The closing test is the fastest evaluation instrument in the room and it costs nothing. It also settles arguments, because the person holding the phone is not performing for anyone.