An easy way to check for sensor dust is to point your camera at a white wall, set a really small aperture (like f22), then manually focus till the wall is just a blur, then take a shot. Helps to isolate the dust specks
On a similar note, how does one clean the prism in a 350d. The bit of plastic/glass between the mirror and eye piece? Somehow I managed to get a few bits of dust on it and in an attempt to clean it with a cotton bud, made it worse