How To Spot The Dust
The distinguishing feature of the dust deposited on the sensor is that the smaller the aperture, the more it becomes visible. The large openings of the spots due to dust tend to be very blurry and very difficult to see. With very very small openings, on the contrary, the spots will be dark and well-defined and therefore clearly visible. The best way to determine how much dust is on the sensor is to take a blurry photograph with a goal to its minimum and with a subject of uniform color Opening you can use the blue sky or a white paper or a wall as a target.
To better see all the debris on the sensor you can increase the contrast of the image, or extend the histogram. Attention that proceeding with this methodology does not only see the dust but also a whole series of further defects of the camera as the vignetting http://bdmaster.net, flares and other problems introduced by the lens. Problems that would not normally be visible in case of normal pictures, so do not worry.
Also the viewfinder is difficult to clean rough being is to avoid any kind of cloth that may leave fibers (the cure would be worse than the disease). The solution is to use a plunger blower, like the one pictured to the right the use of air is in fact the best way to remove dust and avoid scratching or leaving residue. Obviously, in the case where there are stains (even water), all that remains is to use a cloth and specific products the cloth should not leave residues (a chamois leather or cloth to clean the glasses are NOT good solutions) while for the more cercatene of specific products and avoid those intended to glasses because they have inside antifogging agents.
Attention, in the process of dust control, also to the lenses the dirt could also lurk on them, in particular on the front lens. If, however, the latter is able to endure frequent cleaning, the rear lens (the one to the sensor) is certainly more delicate. Internal dust instead is something difficult to eliminate open the lens body could create far greater damage than the dust (or condensate) could generate.
The objective of this cleaning is to get a picture that does not show anything reprehensible when a photograph is taken. Never exaggerate cones cleaning cycles the repeated and excessive cleaning to remove every speck of dust in the long run will damage the camera.