**Core Concept**
The earliest method of diagnosing pituitary tumors involves visualizing the tumor's presence and location within the sella turcica, which can be achieved through computed tomography (CT) scans. This imaging technique provides high-resolution images of the pituitary gland and surrounding structures, allowing for early detection and assessment of tumor size and extent.

**Why the Correct Answer is Right**
CT scans are the earliest method of diagnosing pituitary tumors because they can detect calcifications within the tumor, which are often present in cases of pituitary adenomas. Additionally, CT scans can evaluate the sella turcica and surrounding bone structures, helping to determine the tumor's size and potential impact on surrounding tissues. This information is crucial for planning further diagnostic tests and treatment strategies. CT scans typically involve the use of contrast agents to enhance the visibility of the tumor and surrounding structures.

**Why Each Wrong Option is Incorrect**
**Option B:** Visual field changing is a late manifestation of pituitary tumors, often resulting from the tumor's compression of the optic chiasm or nerves. While visual field defects can provide critical information about the tumor's location and impact, they are not an early method of diagnosis.
**Option C:** Visual evoked potential (VEP) testing is used to assess the function of the optic nerves and can help diagnose conditions affecting the visual pathway. However, it is not a direct method of diagnosing pituitary tumors and may not be sensitive enough to detect early changes.
**Option D:** X-ray skull is not a suitable method for diagnosing pituitary tumors as it cannot provide sufficient detail about the sella turcica and surrounding structures. X-rays are more useful for detecting bony abnormalities or fractures.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that early detection of pituitary tumors is critical for preventing potential complications, such as visual field defects, hypopituitarism, and increased intracranial pressure. A high index of suspicion and prompt use of imaging modalities like CT scans can help identify pituitary tumors at an early stage.

**✓ Correct Answer: A. CT scan**