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You won't see marks on the sensor by looking through the viewfinder as the sensor is hidden behind the mirror, what you see through the viewfinder are marks on the mirror, prism and focusing screen, usually on the inside of the focusing screen. There is a small catch on the screen which makes the holding plate drop down and you can take out the screen and clean it. You can clean the underside of the screen and the mirror when you take the lens off.
As regards sensor dust, stop the lens down to f22, take a shot of something white, like the sky or a door, making sure you get a reasonably bright image, then use that image to check for sensor marks.
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