Dust spots on sensor or lens?
Checking the sensor and lens for spots and proper cleaning:
1. Use anything white to photograph it doesn't matter what as long as it fills the frame and you have a completely white image.
2. Set your camera to the lenses minimum aperture (mine's f.22) and the lower ISO the better.
3. Take your photo and inspect for dust spots. Make sure image is totally white by looking at your histogram.
4. You can buy kits to properly clean the sensor without damaging it and this I highly recommend. Tip: find out the actual size of your sensor beforehand.
5. After you have cleaned the sensor do the steps once again. If you're still seeing dust then you have dust inside your lens and the best way to clean that is to send it off to the manufacturer. (external zoom lenses are the most vulnerable to trapping in dust)