Condition Monitoring for Small Bearings

Insufficient information for "textbook" answer.

Condition assessment and monitoring is a relatively specialist field. One of the first questions asked will be "what is the consequence of failure"?

A small bearing on a wheelbarrow might still be suitable for use at condition 4 while the same bearing might be worthy of replacement at condition 2 in a motor or where replacement is time consuming or problematic (like a ballast pump on a submarine.).

The purists will talk of harmonic vibration analysis, thermographic measurement, lubricant analysis, but it really depends first and foremost on the context of the installed condition and expected operating outcomes.

The cynical response would be "just look at it, that's cheap." but you need to explain context.

On a rough scale of increasing attention:

Listen to it--in air, or with a stick between it and the ear, or with a stethoscope.

Check temperature--by touch, by thermocouple or infrared thermometer, by thermal imager.

Periodic lubricant analysis (in particular for metal particles resulting from wear).

Vibration analysis.

Of course, maintain bearing lubrication per appropriate schedule, accounting for such things as the presence of water that might wash out the grease/oil. Similarly, there may be a replacement schedule for various bearings.