How are the "Extreme pressure" properties of a lubricant measured?

The usual method for characterising a lubricant's "Extreme pressure" properties is to carry out a four-ball test (described by the ISO 20623 standard).
Procedure:

• three balls are firmly held together in a basin
• a fourth ball, on a mandrel mounted onto the vertical driveshaft of an electric motor, applies a variable charge onto the three others
• the charge at which the weld between the 4 balls occurs is measured.

There are other tests for measuring extreme pressure performance, such as the FZG tests, for example.