The way to check if the sensor is dirty is to take a shot on a reasonably evenly colored object. In this case, I used a reasonably even white wall. Any bright color would do.

A good trick is to set the focus to manual mode and set distance to inifinity. Then take the shot very close to be sure that the wall will be completely out of focus. Otherwise, dirt on the surface could disturb the result. Remember that the DOF is large at f22 - for a 100mm lens that I used, at 3 meters focus distance, the DOF is about 1 meter at f22.

A blue sky is good as well - then do the opposite. Set the distance on the nearest and shoot towards the sky