The most common indicator is a grease fitting that doubles as a wear indicator. That fitting, or boss, will stick out of the bottom of the joint’s housing by about half an inch (1.25 centimeters). As it wears the boss will recede into the housing. As long as the boss protrudes out, the joint should be fine. Once the boss is flush with the housing or has receded even further, you’ll need to replace the ball joint.