Worn shocks on a car can make the vehicle unsafe to drive. Many cars nowadays use a strut suspension, which is a shock integrated with the coil spring. When a shock or strut fails, it is unable to settle the rebound motion of the coil spring when going over bumps, braking or accelerating. This creates maneuverability problems with the vehicle. Worn shocks or struts can also prematurely wear other steering and suspension components such as coil springs, tie rods, ball joints and tires.
Test drive the vehicle first. Make sure the test drive road offers a variety of terrain and is free of traffic. A large vacant car park will work well. On the ride to the test drive road or car park, accelerate the engine to notice if the rear of the car squats under duress. This would be a sign of worn rear shocks.
Brake hard once on the test drive road or parking lot--making sure no one is behind you--to determine if the car nosedives severely. This would be indicative of very weak front shocks or struts. The effect of squatting and nosedives on a vehicle can be very dangerous, causing a rocking motion forward to back under normal driving conditions.
Locate some small potholes or bumps to drive over. Clunking noises as the shock or strut bottoms out is a sign of weakness in the component.
Take wide and short turns with the car on the test drive road or car park to determine how the shocks or struts handle on the same axle. Excessive swaying from side to side would indicate weak shocks. After an assessment is made for further inspection of the suspension, drive the vehicle back to the desired location.
Bounce each corner of the vehicle once it is parked on a flat, paved surface. Use an aggressive bouncing motion--being careful not to damage the car--and give each corner three good pushes and then step back to see how the corner of the vehicle handles it. The shocks are designed to prevent the bouncing motion of the coil spring. A good shock will rebound the coil spring after one bounce. Two or more residual bounces is a clear indication of a shock that needs to be replaced.
Place the car on a lift or raise one axle and place the car safely onto jack stands.
Remove the tires using an impact gun and socket to remove the lug nuts. Inspect the tread surface of the tires. Worn shock or struts will begin to show their effects on tires. A feathering or cupping uneven wear along the tread is a telltale sign of poor suspension components. This is caused by the excessive bouncing motion while the car is operating transferring to the tires.
Visually inspect the shaft of the shocks or the struts to look for signs of leaks. A leaking shock or strut may have a wet, oily surface showing clear signs of failure; however, road dust, dirt and other debris can quickly coat the oil leaking from a shock. Compare how one shock looks to the other. If only one shock on the same axle is coated with a thick, black layer of greasy-looking dirt, it's most likely leaking.