To check your camera sensor for dust, here are a few simple steps to take. Set the ISO @ 100 or the lowest setting. Set the aperture to F/22 or higher. Turn off the anti-shake or any other image stabilizing feature. Take a picture of a grey card, a light colored even surface, or blue sky. Move the lens around during the exposure. Be careful not to over-expose the shot or you won't be able to see the dust. Take a few different shots just to be sure. View the images on your computer. Increasing the contrast shows any dust spots.