I suspect there may be dust on my sensor. I have a recommendation for a shop who does a good job of cleaning sensors, but I would like to test myself how much dust there may be. I have read somewhere which kind of picture I should take to make this problem obvious. If I remember well, the subject should be clear and out of focus? Any idea?
Clear, out of focus, so a sky shot, and slightly over-exposed - but try an exposure bracket.
Before giving it over to the pros, have you tried something like this?
http://www.henrys.com/webapp/wcs/stores ... emID=98951
I have a similar contraption and have found that setting it up on my tripod with the body pointing down and a few short shots of air has dislodged everything to date.