elevation calculation: GPS or barometric altimeter

In general, you can very often not know afterwards, which sensors were used for a measurement. For example, you can't see anywhere (unless analyzing the .fit file with third party tools) if the optical HR sensor in the watch was used, or an external HR sensor was used.

For altitude, though, you can be pretty certain that the Fenix 5+ will always use the barometric altimeter. The GPS will only be involved in calibrations of the barometric altimeter. And even then, the GPS will often only be indirectly involved in the calibration, delivering a 2D GPS position to the watch, will then use map altitude for that position.

thank you