"Checking for Dust: There are two methods for checking for dust. You can take a photograph of a clear blue sky or a smooth white wall using a 50mm lens or smaller at f16 or f22. If dust exists it will be seen in the image."
It should be added, that to truly see how bad the dust is, or conversely... how clean your sensor really is, enlarge your image to 100%, then use the hand tool in Photoshop Elements or PS-CS to more around and carefully examine the full sized image on your computer screen.
I have attached a rather, unhum, embarrassingly dirty image from a shoot in France's Loire Valley this summer. The first is full frame, 900 pixels wide, with only RAW processing performed, then a straight conversion to an srgb JPEG file. At first look, it appears to be pretty clean.
The awe... what alotaCRAP! moment comes, when you enlarge to 100% and discover, just how bad it can get, as seen in the second image of the area above the towers.
I had thoroughly cleaned my sensor the evening before. All it took was a few lens changes during the day, for those dirty rascals to sneak in! The little buggers!