If you know someone who will lend you a G Meter, swap out one of your other instruments for the flight(s). I found it much easier to nail this using the G Meter than with other methods.
The second best method was to take a passenger, have them hold a tennis ball and tell you when it starts to float. As others have said, that is when you release the red button.
Depending on where you mounted your buttons, it can be quite a trick to hold the button, push with just the right pressure on the stick, while simultaneously watching for something to float.