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Set camera to lowest ISO and aperture-priority to highest-numbered f-stop (e.g. f/16, f/22, ...). Set lens to its highest zoom and manual focus to infinity. From close enough so that only white fills the viewfinder, aim at white of screen and take a metered exposure. In Lightroom develop window or the like, view at 1:1 and bump 'Exposure' and 'Blacks' up until dust shadows stand out clearly and you become suitably depressed at the dirty state of your sensor.