How to know if underbite is jaw or teeth?

Orthodontist: With a special jaw x-ray , a lateral cephalometric film, coupled with models of your teeth and a thorough clinical examination, a qualified orthodontic specialist can give you a definative answer.

Exam and records: Comprehensive orthodontic records including head x-rays(cephalometric film), jaw joint x-rays(cbct or i-cat), study casts, and a comprehensive exam by a qualified orthodontist will provide the answer you are seeking.

Dentist: Can be either. An orthodontist can give you a comprehensive exam, including appropriate x-rays and treatment plan.

See an orthodontist: for examination and records to determine if is skeletal or the lower teeth being positioned further forward. If the patient is an adult, and the underbite is skeletal, surgery is usually required. See your orthodontist.