Before trying to assess for volume loss (or for anything else for that matter), what should you do?
Assess the technical adequacy of the film.
Is this film rotated? If so in what direction?
Yes. To the right (look at the medial clavicles compared to the spinous processes).
What is the differential for the left upper zone opacity?
Pulmonary mass (cancer, metastasis, hamartoma), pseudotumour. Abscess / consolidation / round atelectasis are all unlikely given how sharply demarcated the margins are.
How can you work out which of these it is, and determine it's location?
A lateral chest x-ray may show that this lesion is located within the oblique fissure, however in an unwell and large patient they are often difficult to adequately penetrate, especially in the upper zones. In this instance the patient had had a CT chest a few days earlier.
Chest x-ray (rotated to the right) demonstrates widespread airspace opacity (possibly with early cavitation) in the right mid zone. A large left sided pleural effusion is present with associated atelectasis. Nasogastric tube and ECG leads noted.
In the left upper zone a well circumscribed opacity is visible.