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How do I spot dirt/dust on the sensor?

With system cameras with interchangeable lenses, it is unavoidable that the sensor will get dirty—whether that is dust from the environment, oil from moving parts (like the mirror in an SLR camera), fibers from a cleaning brush/cloth or an unintentional fingerprint. It happens to us all, although you do not see it on every picture. Even if you never change lenses, there is a chance of the sensor getting dirty. A small number of the—generally more expensive—cameras and lenses are extra-well sealed against dust. Even that is no guarantee that no dust will reach the sensor. Dust is everywhere. If you use a zoom lens that changes length when zooming in and out, then you also suck new air—and thus dust—into the camera. The question is thus not whether your sensor will get dirty, but when your sensor will get so dirty that you will start to see it.
How do you recognize dust and other dirt on a camera sensor? Someone who wants to buy a second-hand camera should certainly know the answer to this question. How do you clean a sensor, and how do you prevent it from getting dirty again?