I was seeing Kaiser’s regional corneal specialist for epithelial basement membrane dystrophy disease when I developed sudden onset PMR. On my next appointment my ophthalmologist reassured me I did not have inflammation/GCA. Then on our next evaluation he diagnosed inflammation in my eyes from allergies; not GCA. Bottom line, there is a way to check for inflammation of the optic nerve by a skilled ophthalmologist.

I was seeing an ophthalmologist regularly for uveitis before PMR was diagnosed. He saw inflammation inside my eye all the time. When PMR was diagnosed, my ophthalmologist would also check for "evidence" of GCA. I'm not sure what he was looking at but he always reassured me that he didn't see any evidence of GCA.
https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/uveitis#:~:text=Uveitis%20is%20inflammation
I guess an ophthalmologist can visualize the optic nerve. When it is being deprived of blood flow then it changes. I was always on a relatively high dose of prednisone for PMR anyway so I stopped worrying about GCA.