Your sensor will contain dust, eventually. Even self cleaning sensors eventually do collect dust. It looks like this:
To actually see this dust, as in the image above:
- Use BULB mode
- f/22 (smallest aperture)
- Wider angle
- Aim at a white surface, out of focus (use manual focus for this)
- Click and wave the camera about. Try an exposure of 3 seconds. If too dark, make it 65. If too light, make it two. And so on.
Once you have the image, that shows you the dust you have.
If you need to clean, there are solutions (pads, brushes), but use them with care. Start with a blower, then use the brushes, then use wet pads. A photo store can tell you the pros and cons. Risky, maybe – but I have been doing it for years with no ill effects.