Several ways.

1 - You can leave the plane on the ground and twiddle the controls until the power runs down, then back off a little from the measured time.

2 - You can turn off your plane and plug in the voltmeter with load, and watch to see how long it takes to drop to the unsafe area.

3 - You can measure the typical draw of the electronics with a meter, then enter this as a discharge rate quantity into an intelligent charger/cycler, then let the cycler tell you how long the battery will last.

4 - You can measure battery capacity with the intelligent charger, and divide that by the average draw to give you an expected run time.

5 - You can fly the plane and take a measurement after each flight, using the voltmeter. Then once the plane gets into the danger zone use the voltmeter alone to calculate the remaining possible flight time.

etc.

Much like determining what to set your TX's timer to w/o first crashing your plane by letting it run out of fuel.