Would a x-ray or a cat-scan be used to determine an abnormal skull shape?

Possibly: An abnormally shaped head in infants can generally be determined by physical examination. In instances of premature closure of the skull bones sutures(craniosynostosis), either skull x-rays or ct scan may be used to define which sutures are prematurely closed.

Either: The plain x-ray can give some notion. The ct scan will provide the 3-d anatomy.

Neither: The specialists at the craniofacial center at Medical City Dallas use physical exam. If they want to confirm an issue or satisfy the curious they might use either, but a physical exam and measurements can easily separate the normal from not. If they are planning to remodel the skull, they get a specialized detailed CT with extra features.