I just bought a Canon dSLR camera with an 18-135mm IS lens. I really want to take care of it, as I paid extra for this "better" lens, so can the lens get, in some way, damaged if you drop it, say 5 cm, by accident? Is there any test that can be made to see whether or not it was damaged?
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:
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.
Dropping the lens accidentally on a ground with stones etc. can do a terrible things. As the lenses are really expensive (unluckily I am not a millionaire), I bought a good carrying camera case. I already had a bad experience with some Vanguard "pistol" case. I accidentally slipped on a greasy road and although I tried to protect the camera, I found that the front lens UV filter is cracked.
So I bought new case (CaseLogic) with more protected bottom (this new plastic shell on the bottom) and also a new UV filter. I use these filters only for protection - it is good - you do not have to care much about fingerprints, very useful also near sea (the salt water is also problem).