How can I easily tell if a sensor or lens is damaged?
I am pretty interested in recognizing any damage to digital camera in very low level (without any advanced tests).
For example, how can I identify the damage of a sensor or lens? If there are any ways to identify that the camera was dropped or physically damaged. I am pretty sure we can say it based on photo, or may be camera has any kind of sensor.
This is a good place to start on how to test a lens for damage http://photo.stackexchange.com/questions/12215/how-can-i-test-a-new-lens-to-make-sure-it-is-operating-correctly
It sounds like you are wondering if cameras have a "black box" of sorts. http://en.wikipedia.org/wiki/Black_box_%28transportation%29
Unless there is obvious or visible damage or dysfunction, minor issues can be quite hard to determine. You can severely compromise the front element of a lens and it will still mostly work. However a scratch on a rear element will be much more noticeable. But you can have dust in a lens or camera body with little or no noticeable effects.
If you have the ability to see images produced by a lens or camera body you can look for some things such as dark spots, which may indicate dust or dirt on the lens, or banding which may indicate some deeper malfunction. For a lens, the most common problems show up as poor focus, or uneven focus across an image.
If you are asking before you buy, just make sure you have some reasonable amount of trust in who you are purchasing from. In general, photography equipment holds up very well, if taken reasoanble care of.