Don't need a sky nearby to do a quick check of your sensor for cleanliness, a uniform color wall will do. Just set the room light to allow a normal exposure to be in the multiple seconds neighborhood, and take a picture of any reasonably uniform surface (I use a white wall), "swaying" the color around during the exposure to blur out any detail. The dust will move with the camera, of course.
Everybody whips out the "sensor swab" method whenever sensor cleaning is mentioned, but in my own experience, the first line of defense is to try to clear off the sensor with air. Either use CLEAN compressed air, like www.americanrecorder.com
, or a big bulb blower like a Giottos Rocket. Never use regular office supply "dust off" products, they can be "goopy", and never use a bulb blower brush directly on the sensor, unless you're trying to use the sensor to clean the brush.
If this doesn't do the job, then resort to the "spatu-swab" technique. If it's the first time you've ever done this, and you've had the camera for a while, the sensor may very well be so dirty that you have to use the swab technique any way.
-harry