I believe the gold standard is a 3D CT scan. I just read this in an article about Eagles: Three-dimensional computed tomography (3D-CT) has several advantages over conventional coronal and axial CT images, including providing accurate anatomical images and the relation between adjacent tissues, and is thus potentially useful for the diagnosis of Eagles syndrome. A number of groups have advocated the use of 3D-CT for radiologic examination of patients with Eagle's syndrome.
But people have been diagnosed with regular CT scans, CT scans with contrast, and panoramic x-rays.