Reactive arthritis was diagnosed 20 years before PMR was diagnosed. The main reason PMR was diagnosed was because I couldn't lift my arms without shoulder pain. That was something I had not experienced before. My rheumatologist first asked me if I had joint pain or muscle pain. I was unable to answer because I couldn't distinguish where the pain was coming from or what was causing the pain.

The next question was whether or not the pain was anything I had ever experienced with reactive arthritis. I immediately told my rheumatologist the shoulder pain was completely different. That was basically how PMR was diagnosed.

The difficulty I had being diagnosed with PMR was that I was already taking prednisone. My rheumatologist wanted me to stop prednisone so she could assess my symptoms when I was off prednisone. I could only stay off prednisone for a couple of days but my rheumatologist saw what she needed to see. She said I had both PMR and reactive arthritis. There were two rheumatologists who were evaluating me at the same time. One rheumatologist thought it was a flare of reactive arthritis and didn't want to prescribe prednisone. The other felt strongly that I had developed PMR in addition to reactive arthritis and reluctantly said I would need prednisone for a long time.

Some scans will show where the inflammation is but people with any type of spondyloarthritis will have inflammation in similar places as PMR.
https://rheumatology.org/patients/spondyloarthritis
Except for the spine, most patients with PMR will have inflammation in shoulders and hips rather than the spine.

It takes time for the damage caused by inflammatory arthritis to show up. In the beginning, it is hard to differentiate between PMR and spondyloarthritis. It is common for people who were diagnosed with PMR to have their diagnosis changed to spondyloarthris later on. The reverse is also true, people with spondyloarthitis have their diagnosis changed to PMR. Much of this is determined by age until the arthritic damage starts to show up.

The diagnosis isn't as easy as people think. A rapid response to prednisone is one criteria for PMR but spondyloarthritis and late onset RA (LORA) responds the same way to prednisone.

I think you might have PMR but my opinion doesn't count for much. I'm not a doctor. However, I wouldn't want you to go down the road of long term prednisone use either.

I just hope your rheumatolgist gets it right before prescribing prednisone for PMR. IF your IL-6 level is elevated that might be more evidence for PMR. With any luck you might be treated with Actemra or Kevzara instead of prednisone.