How to check the camera sensor at home.
It is 2017, we don't need blue sky for checking of the sensor anymore.
Use lens with smallest aperture you have. Set the lens to the smallest aperture. Turn on the table lamp. Make sure it is the naked bulb, no fancy covers. Take white paper for printer and place on the lens front or lens hood. With ISO set to 400 or less, shutter speed on auto, point the lens to the lamp. You'll get something like 1/15 for shutter speed.
Just hold it while image is taken. It doesn't matter how steady it is going to be. Where focus is doesn't matter either. All you are going to see is white paper, make sure it is evenly exposed, by checking it in VF before and while the image of the sensor surface is taken.
This method works with any camera, not just digi M Leica.