Yes, they do unload in flight. As noted above how much depends on your set up and flying style. If you do 3D hovering then you will load the motor up just as much, maybe more, but for normal flying where you are moving forward then the prop does unload and amps drop significantly.
Also bear in mind that once the initial 'fresh off the charger' burst has warn off (a few seconds) amps will drop a bit anyway. Providing that the motor never gets so hot that it becomes uncomfortable to touch for a few seconds then you should be fine.
It's far from impossible to measure this. You just need an ESC that has data logging or a add-on data logger.. these are quite common these days.