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Look for a wear indicator. Raise the front end of the car. Rock the wheels and check for play.
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. 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. This applies to older cars. Most new cars do not have wear indicators, or a grease fitting. If you can’t find a wear indicator, or if the indicator doesn’t give you a clear enough sign, raise up the car to inspect the joint more thoroughly. To check a loaded lower ball joint, the manufacturer will recommend that you place a jack under the lower control arm of the front wheel, as close to the ball joint as you can, then raise the vehicle until the wheel leaves the ground. If you don't do this, tension will still be on the ball joint when you jack up the car, making it difficult to move and look for play on the joint. Make sure there is no compression in the suspension when you check the ball joint. If you have control arms of unequal lengths, make sure the upper bump stop doesn’t touch the control arm. Make sure your car is jacked securely. The vehicle must be on frame stands when jacked up and you are working on the vehicle. Move dial indicator to the side of the nut or to the side of the spindle and move the wheel in and out to check radial wear.  For vertical play, place the dial indicator against the steering knuckle stud nut or the ball joint housing, depending on the manufacturer. Grab the wheel at the top and bottom (12 and 6 o’clock) to lift up and pull down. Move the dial gauge to the side of the nut and grab the tire at the bottom and move it in and out to check radial play of the joint. Read the dial indicator and check against the manufacturer’s specifications. If the movement is beyond specifications, replace the ball joint. Experienced mechanics will also just check play by the feel, listening closely for any clicking or easy motion that suggests the ball joint is worn out. Alternatively, you could have a helper look closely at the joint itself to see if it appears loose as you move it.