It will show up as corrosion or some type of physical damage. Too much shaft radial movement, whip, wobble or run out is a common cause of a rotating seal component hitting a stationary part of the pump or a rotating shaft hitting a stationary seal component. When a rotating component hits a stationary component you will see evidence of rubbing or damage on both parts. Cavitation means random and excessive radial and axial movement.
Harmonic vibration occurs when the seal is vibrating in harmony with another piece of equipment.
Look for this problem with the seals in standby pumps. You will see lug wear and carbon chipping on the outside diameter of the carbon face. Hitting a critical speed. This happens with variable frequency motors and pumps designed with flexible shafts. The pump pedestal does not have enough mass to support the pump and its driver.