The easiest way to test a lens for damage is to take a photograph using the lens and look for any unusual softness (either all over, or uneven sharpness across the frame), lack of contrast or other striking defects that weren't there before.

You might also notice stiffness or restrictions in the zoom or focus ring operation that can indicate damage, or failure of systems such as autofocus or image stabilisation.

Yes, lenses can be damaged by impact, the likelihood of this occurring depends on:

• The height the lens was dropped from. 5cm isn't all that high so the lens wont be going too fast.
• The surface dropped onto. A soft surface will slow a falling lens more gradually and be less likely to cause damage.
• The build quality of a lens. Some high end metal body lenses are designed to take knocks. I dropped my 135 f/2.0L onto a hard surface from waist height and it was fine, though see the next point.
• Luck.

By the same token lenses can be damaged when other objects come into contact with them.

The other common cause of damage to lenses is moisture. Either getting the lens excessively wet during a rainstorm, or storing it in damp conditions which allow mould to develop inside the lens. Again some high end lenses are designed to handle wet conditions.

You can't really use a lens without putting it in harms way, so unless you leave it at home wrapped in cotton wool, it may get damaged. By far the best way to protect your lens is taking out insurance. The latter will also protect against theft, you might even be covered under your household insurance so check the policy.