How to test if a shock absorber is good

On many occasions when we have a used shock absorber in our hands we do not know how to determine if it is in good condition and its replacement is necessary.
First we must look for obvious signs of damage: drips or leaks of oil or hydraulic fluid, tube breaks, oxidation, cracks, wear, which compromise the operation of the shock absorber.
SMELL OF BURNED OIL coming from the shock absorber is sufficient indication to replace the shock absorber, especially if the shock absorber has more than 50,000 km of travel.
A good shock will always prevent its axle from moving easily, so we can apply force to the axle and feel how it moves: if it moves easily in either direction then the shock is bad and should be replaced.
If applying force to the shaft produces metallic sounds and/or metallic knocks then the shock absorber is damaged.
Repairs to the shock absorber are neither possible nor advisable.