Chest x-rays are generally taken when the patient is in the inspiratory phase of the respiration cycle, in layman's termed having breathed in. This has an important effect on the quality of the x-ray. When the x-ray beams pass through the anterior chest onto the film, it is the ribs closest to the film, the posterior ribs, that are the most apparent. You should be able to view ten posterior ribs if it was taken under full inspiration. If you can see 6 anterior ribs, then the film is of a very high standard. Overexposed films look darker than normal, and fine details are very difficult to see. Underexposed films look whiter than normal, and cause the appearance of areas of opacification. Look for intervertebral bodies in a properly penetrated chest x-ray.  An under-penetrated chest x-ray cannot differentiate the vertebral bodies from the intervertebral spaces. It is under penetrated if you can't see the thoracic vertebrae. An over-penetrated film shows the intervertebral spaces very distinctly. If the patient was not completely flat against the cassette, there may be some rotation evident on the x-ray. If this has happened the mediastinum can look very unusual. You can check for rotation by looking at clavicular heads and thoracic vertebral bodies.  Check that the thoracic spine aligns in the centre of the sternum and between the clavicles. Check if the clavicles are level.

Summary:
Check if the film was taken under full inspiration. Check the exposure. Check for rotation.