How Do You Know if Your DSLR Sensor/Filter Needs Cleaning

On some landscape shots I've taken recently I've noticed some significant spotting on the resulting images. I've cleaned my lenses and the spotting remains, so it is most likely an issue on the camera sensor (actually the filter in front of the sensor). I followed Bob Atkins' advice and took some shots that would neutralize any lens effects and repeated with a separate lens. The results are clear - I need to do some cleaning.

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