On Thursday, March 19, 2020 at 9:33:44 AM UTC-7, wrote:
"How do you determine the tail is lifting in gliding flight?"

Agree. Do we have wind tunnel testing to prove the theory that your fixed stab is always providing upwards forces on the aft end of the fuselage?

I used do some free flight modeling.
Mostly Dick Mathis designs.

There is a misconception that a requirement for stability is that the tail must be providing a downward load. In fact all that is required is that the lift slope of the tail be higher than the wing with increasing angle of attack. That guarantees a righting moment with any angle of attack disturbance.

All this talk about free flight models reminds me of family connections. My brother won the world competition in F1B rubber powered free flight a few years back (first US win since the mid '50s). These aircraft have surprisingly sophisticated powertrains using secret and carefully hoarded rubber motors driving folding, feathering, torque sensing prop hubs. The latest have wings that are folded in half for the high speed climb, and unfold for the glide. They typically have lifting tails.