RE: lipo batteries
2 other ways: (if lipo pack)
Check the remaining capacity after the flight by resting voltage. Kinda the same as barracuda said , but with out the re-charge. You would've still had to time the flight and check the voltage.
Another way is to measure the current average and divide that into the capacity that you wish to use up. It may be somewhat difficult to measure the average current on the ground though.