I stumbled on a way to examine my Z 6 sensor for dust.
- Close down the lens (I was using a F 50 f/1.8) to it's smallest aperture.
- Aim it at a clean white surface in bright even light (I used the back of a 13x19 photo paper) and play with the exposure.
- Look through the viewfinder, magnify of you need to.
- Dust blobs, if any, will show.
- Clean the sensor using a rocket blaster or the built in sensor shaker) and check again.
- No need for downloading and playing with software.