From a practical view point, watching radio controlled gliders, I was
able
to take advantage of that. It would tell me when I entered a thermal, the
fuselage boom would tilt up and when I fell out of one, it would go down.
I noticed that the model glider would stay in an accelerate state after is
stabilized in the thermal In a stick fixed position,
The model would have to be set up similar to a free flight model, so as not
to crash if no control input is given. To take full advantage of the thermal
I would have to use up elevator to maximize the climb and the speed would
be reduced of course, not unlike in the full size gliders.
That would indicate that the relative plane relative to the horizon has
shifted due to the thermal. The glider wants to fly "more down hill" if no
other
input is given.

Is the glider simply weathervaning around the lateral (pitch) axis due to
upward pressure on the horizontal stabilizer? Once in the thermal the lift
is affecting both the wing and fuselage and tail, but might the arm of the
horiz stab cause the whole glider to rotate around the CG, i.e. nosing down?
We would feel that as more than just an upward kick in the seat, also as a
slight tilt forward. If the tilt persists, the airspeed increases. You
report seeing the boom tilt but don't see which way... I seem to recall my
R/C glider nosing up first, as the wing enters the thermal first, but maybe
the tail overcomes it a second later - I don't remember.