Flashman did a really good review of the arctic butterfly a couple of weeks ago.
Shoot through a lens wide open at a white piece of paper. Downlad the pic and blow it up. You can then see the dust more easily.
I cleaned the sensor on my 20D for the first time last week using something called dust aid but there are plently of other good product. Flashman did a really good review of the arctic butterfly a couple of weeks ago.
Hope that helps.