Larger radials had torquemeters- usually measuring (via a hydraulic servo) the reaction of a stationary ring gear in the planetary set. In such an installation, you flew by torque reading, and the manifold pressure would gradually creep up as rings and valves deteriorated after many hours.
Modern turboshafts/turboprops also have torquemeters - in that case torque is the output, and TGT/EGT gradually rises with airfoil & seal deterioration.