[header] How to check ball joints [title] Look for a wear indicator. [step] While the vehicle is still on the ground, look underneath the car, on the bottom of the steering knuckle attached to the wheel. 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.