How do physicians test the diameter of my aorta?

Ultrasound, CT ABD. Either ultrasound or ct of the abdomen will reliably identify abdominal aortic aneurysms (aaas). The ultrasound is ideal for screening because it is inexpensive and has no radiation exposure. Ct is better at defining the precise location of the aneurysm to help decide the best technique for repair.
Various ways. Can be measured directly while seen on ultrasound, ct or angiogram. Initially starts with physical exam. Some groups encouraging ultrasound screening in high-risk patients. Others seen incidentally on scans for other reasons.
AAA. Above 3 cm diameter is an aneurysm. Above 5 cm increased risk of rupture so treatment needed. Usually an endograft. If you have AAA need ultrasound every 6-12 months.