How to check your sensor
1. Find a large flat surface of bright uniform colour. I use my white ceilings.
2. Set the camera in manual mode and lens in manual focus.
3. Set a small aperture, e.g. f/16
4. Set the shutter speed slow enough to overexpose by about +1 or +2 EV.
5. Point your camera at the surface. Get close to it, but not too close, you don't want a shadow.
6. Focus the lens at infinity, you want to blur out any specs on the surface.
7. Take a picture.
You should get a bright uniform smooth area in your picture. You will see any dust specs from the sensor. The small aperture makes them sharper and easier to spot.
How to clean your sensor
I use
Eyelead gel stick (available on Amazon). Very good German engineering. Doesn't use any liquid. It is a piece of sticky gel that grabs all the dust from the surface of the sensor. Then, the dirt is transferred to a piece of sticky paper. The idea is that the paper is more sticky than the gel, so it collects the dust from the gel, not he other way around. After this the paper can be disposed of and the gel reused.