How do you verify the 'Preload' of a wave spring in a blind assembly where direct measurement is impossible?

In blind assemblies, preload is verified indirectly through 'Stack-up Analysis' and 'Torque-to-Turn' measurements. First, a statistical tolerance stack-up (RSS method) is performed on all components (housing depth, bearing width, spring free height). Second, because $P = k \times f$, the axial force is related to the frictional torque in a rotating assembly. By measuring the torque required to rotate the shaft, engineers can correlate it back to the axial preload $P$ using the coefficient of friction $\mu$. Alternatively, ultrasonic pulse-echo techniques can be used to measure the compressed height of the spring through the housing wall in highly critical aerospace applications.