One of the most useful symptoms I've found - which warns, but does not prevent the 'event' is someone going quiet on you or becoming disinterested.
One trail lesson student was having a great time but then, as we approached the airfield, he went quiet and I asked: 'Are you feeling ok?'.
The prompt reply was: 'I feel BLAHHHHH' - all over the cockpit!
I always assume that trial lesson students will be excited, have just eaten and be looking all around taking in the sights and perhaps moving their heads up and down adjusting cameras and the like - all aids to motion sickness!!!
So I talk to them (to ensure they're not 'going quiet) and limit all turns to rate 1 (15 degrees angle of bank) and fly as smoothly as possible. Try not to surprise (and therefore alarm) them by doing something unexpected (even closing the throttle or lowering the flaps can induce anxiety).
In 18 years of instructing I've only had two students be sick - which I think is lucky. I'm not saying it's my flying or manner, just that I try and avoid the obvious pitfalls - and always have the fresh air vents partially open all the time come rain or shine.
Hopefully not
SITW
