As early as the 1980s, NASA conducted a large series of tests with the aim of using various technologies to monitor roller bearings.
The performance tests were run on several identical roller bearings under a variety of load, speed, temperature and lubrication conditions.
Bearing temperature, torque, vibration, noise, strain, cage speed, etc., were monitored to establish those measurements most suitable as indicators of roller bearing health.
An artificial flow was created across the inner race surface of one bearing using an acid etch technique to produce the "scratch." Tape records obtained before and after established a "characteristic" frequence response that identifies the presence of the flow.
Bearing performance was monitored and recorded using electronic speed, sound, vibration, strobe-optic, thermal load cell (strain gages) and ultrasonic sensors. Recorded data was correlated with known test conditions to provide elements of a diagnostic system to predict impending bearing failure or distress prior to onset of audible or thermal indications.
Main result of the study: Of the signals recorded, ultrasonic signals were the most suitable for detecting deterioration in the performance of roller bearings.
The whole study can be viewed here: https://ntrs.nasa.gov/citations/19720020851