My technique to check my camera sensor for the presence of dust.
1. A white paper sheet.
1. Set the white sheet on a table.
2. Mark a black spot(with a marker) on the sheet.
3. Fix the camera on the tripod.
4. Set the camera to it's minimum aperture(for mine f22).
5. Focus on the black dot and take the picture.
6. Observe the picture, if it contains random black dots other than the dot on the sheet, it means that there is dust on the camera sensor or the mirror and it needs cleaning.