Subman, I work for a bearing manufacturer. The only way to tell if a bearing is bad is by either (1) A good, calibrated ear and feel, (2) A diagnostic tool devoted to detecting bearing noise, (3) A good visual inspection.

When you take the bearings apart, if they are anything less than perfect, they must be replaced. Most common defect is a spall. Other things to look for are tiny (or large) dents in the raceway, corrosion, brinelling, raceway frosting, bent or broken cage, or discoloration (blue or black) from heat. Any of these conditions require bearing replacement. Bearings must be replaced as a set, not just one race. They must be kept as a matched set (no mixing parts among like bearings). If one wheel has a bad bearing, it's not necessary to replace the bearing on the opposite wheel if it's still good.

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95 E320 Cabriolet, 159K