For rolling element bearings, vibration monitoring (including analysis of spectral content... mcuh more than overall magnitude) can be very effective to detect defects on races and rolling elements.
Vibration can also to some extent identify bearing stressors (unbalance, misalignment)
Temperature is a good indicator of the lubrication performance.
Oil analysis can provide some information on condition of oil-lubricated bearings (for example, wear particle content).
In some cases bearing clearance can be measured while shutdown. Plastigage measurement of bearing clearance in sleeve bearings. In limited cases measurement of shaft movement can be used to provide some indication of bearing clearance.
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