1) Set the aperture at f/22 (any shutter speed will do). If you have a macro lens, set it at f/32.
2) Set your lens to manual focus then focus at infinity
3) Fill the frame with this sky blue image and take the shot
4) Use exposure compensation or change the speed to avoid blown highlights
5) Bring the test shot into Photoshop and view at 100% to see the dust on your sensor. Use auto-levels if necessary.
6) Have a stiff drink.
7) Buy sensor cleaning supplies
8) Have another stiff drink.