Determining Propeller Pitch Angle During Flight

"Actually, isn't the RPM and MAP an indicator of how efficiently the engine is running?"

Yes however the propeller can be considered to be an engine speed governor ie has a direct influence on the engine, particularly rpm & inlet manifold vacuum.

Much like we infer fuel flow, from fuel pressure, we infer propeller pitch (fine - course) from rpm/vacuum.

Knowing the actual pitch may be of interest but will have little practical benefit, above rpm/vacuum, in the effective management of your propeller.😈