Flaking metal
When metal appears to be flaking off across the full surface of the race, you can suspect there is a lack of proper lubrication.

Lack of lubrication can be caused by simply too little lubricant present, or by breakdown of the lubricant from heat or contamination. It can also be the result of using the wrong lubricant -- type or viscosity.

Discoloration
Discolored steel anywhere, on either the cup or cone, indicates overheating. This photograph shows the characteristic look of overheated steel on the outside of the cup.

The primary suspect should be lack of lubrication. Low levels from missed maintenance or a leaking seal should be considered likely candidates.