Pubic ramus fractures can be diagnosed on AP pelvis x-ray. If the fracture is near the acetabulum, additional (Judet) views help to determine whether the acetabulum is involved. Judet views are images obtained by log rolling the patient 45° to each side. If the acetabulum is involved, then this should be managed as an acetabulum fracture and not a simple pubic ramus fracture.
If there is no fracture on x rays but clinical suspicion is still high, CT or MRI would make the diagnosis. Of note, 2-9% of hip fractures are occult (the X-rays are negative). The role of CT and MRI is mainly to make sure a fracture of the acetabulum or femoral neck is not missed, since these are often managed operatively.