Can an outside observer tell if an aircraft is flying through turbulence?
The wings are flexing because there is a change in lift.
The wings are more aerodynamic than the body of the aircraft. I.e. the wings are more "responsive", they respond to changed aerodynamic forces more rapidly.
As a result of the change in lift in the wings, the body (being connected to the wings) follow the change.


Therefore, to an observer who is immune to turbulence, the wings would flex first, followed by the aircraft as a whole. If the turbulence is causing the wings to flex up and down repeatedly (e.g. imagine sine wave), the body of the aircraft can be observed lagging behind the wings and smaller in amplitude.