There are as many methods for testing as there are kinds of dust. I take a 200mm lens set to f/22, focus it to infinity with autofocus off, set the exposure compensation to +1.5, and then photograph my computer monitor displaying a blank white page. Any nonreflective light surface properly lit works — even the sky — but I find the dust really stands out when shooting a monitor image. The small aperture focuses the light rays from the lens to the sensor parallel to each other and causes any dust on the filter to cast a more direct visible shadow.
Then, I view the image in Photoshop at 100% magnification and scroll through the image to look for spots, especially around the edges. Small spots visible in a test image at f/22 may be invisible at f/8 or wider, and I may shoot another test at a wider aperture to check. Depending on the severity or amount of dust I find on this test, I can decide whether it's time for a sensor cleaning or if I can live with it for a while longer.
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