The job of the shock absorber or strut insert is to dampen the oscillations or bouncing caused by suspension coils as they pass over bumps in the road. Testing a shock absorber's ability to return to normal is done by pressing downward on the bumper of the vehicle with the suspected shock. Be careful not to damage the bumper.
A good shock absorber will return to its normal place and come to rest. A bad shock or strut would continue to bounce. This is because the weak shock absorber allows the coil spring to continue to oscillate.