Go flying on the smoothest, most calm day you can find. In level flight, progressively & slowly reduce power to reduce speed until level flight can only just be maintained. Any slower and more power will be required to maintain levle flight so the aircraft must be flying at Vmp.
Or even more simply, slowly reduce the fuel flow (by whatever means) until level flight can just be maintained. At the lowest possible fuel flow rate, your fuel will last for the longest possible time.