There are a couple of ways. A bone scan will show you if you have arthritis. I had a full body scan done a year ago. It was the coolest thing ever to see my skeletal system like that! Not sure what it costs. It's free up here in Canada.
The other method is to have an xray of your body. That's a lot of radiation. LOL. But it will diagnose you perfectly. It's how I got my arthritis diagnosis for my neck. Probably the cheapest option.
Arthritis and Fibro together BLOW. My neck is so buggered up from both issues, I feel like I'm going to lose my mind sometimes. I really hope you don't have it elsewhere. I find that PT works well for me. I'm going back for more sessions soon.
I wish you well!