Set your camera to manual or aperture-priority and select minimum aperture, such as f/16 or f/22, then take a picture of the sky.
After downloading the file to your computer, use your favourite image editing software and crank the picture’s contrast level way up, that is increase the contrast.
The image above is what I just got with my little Panasonic GF2. See all the spots? It’s badly in need of sensor cleaning.