MRI on your whole spine. There's a lot of other things that go a long with it for a diagnosis...as far as me..I have iritis, my tendon tore off the bone of my foot and took a piece of bone with it (I didn't injure my foot, it just happened), plantar faciaitis, colitis, chronic fatigue.
You can also get bloodwork to check for the HLA-B27 test.

Yea, unfortunately there is no one test. I've had back pain & Sciatica for 20 years. Had many MRIs, PT, etc. It wasn't until I had an eye infection (Iritis/Uveitis) and a really good ophthalmologist that I got referred to a Rheumatologist. My Rheum took an extensive hour-long history and examination. Gave me orders for chest & pelvic x-rays. Also, a large panel of blood tests. (they took 12 vials of blood) Everything came back normal but based on my history and symptoms he said I was a textbook case of "Undifferentiated Spondylitis" which he said is when your bone hasn't started fusing yet. My brother has it as well. I'm negative for HLA-B27 but he is positive. I have pain in most of my joints, lower back, hips, iliac crest, ribs, neck, feet. It's caused by Enthesitis. Inflammation of your connective tissue. It happens in most places where muscle connects to bone. He spent an hour during the follow-up explaining the disease and treatments to me. Started on Enbrel 6 weeks ago. (takes 3 months to tell if it works) I seem to be having a Flair now & Rheum just put me on a 6 day run of steroids. This is just one in a whole family of Autoimmune Spondyloarthopathies. I've learned far too much about this disease in the last several months.

Genetic testing, xray of the sacrum, and even if the xrays are negative, you can still have it.
Symptoms, imaging of the sacrum, and the genetic test. My brother has it, Mom has it and my aunts and uncle. Everyone on my mothers side has this horrid spondylitis.

I believe as far as I can see in medical history there is no known connection, however many with spondylitis do have a fibromyalgia diagnosis as well, leading me to believe it's possible in the future they may possibly find a connection.

If you are tested with the hbla thingy it doesn't mean you have it or dont have it... it doesn't always prove anything. The best proof I've heard from many doctors and rheumatologists is patients symptoms and MRI.