To fully evaluate the kneecap and its relationship to the femur below it, one needs to look at the bones from above. It is easy to relate to this view, as it is the view you yourself get of the knees when sitting down.
When the knee is bent, the undersurface of the patella (kneecap) lies in the trochlear groove. The patella should be central in the groove, and not favouring one or other side, and it should not be tilted. The sides of the patella and the walls of the groove should be almost parallel.
Also this is the view taken when 'Merchant View' ('sunrise view') x-rays are taken to show the patellofemoral joint.