There are a few ways to test ball joints. First, listen for abnormal noises coming from the suspension. You can also take a look underneath the car. If the rubber boots that encase the joints are missing or torn, it's a good bet that the joints need to be replaced. Excessive fluid leakage or the presence of rust dust could also indicate a problem. If a visual inspection produces no results, jack the car up and move the ball joint around with your hands. There should be a minimum of play.