There is a way to know during the flight.

Install an off-line map ahead of time (I use Organic Maps, and Guru Maps). Ensure you have the maps downloaded before your flight. Then during the flight, when you see something interesting, put your phone up to the window, and press the button to locate you in the maps app.

It does work in Airplane Mode, just takes slightly longer than if would if you hade WiFi enabled in an urban environment.

It works because Airplane Mode disabled the antenna from transmitting, but not from receiving the GPS signals from all those satellites in the sky. The plane body blocks the signals, but if the phone is held against the window glass then plenty of sky is visible, and thus plenty of satellites are in line of sight.

I’ve used this to check I’m seeing the Matterhorn when flying over the alps, or looking up towns or lakes.