So there is a genetic marker they can look for - HLA-B27 - but lots of people have the marker who don’t go on to develop the condition, and there are people (like me!) who have the diagnosis but are negative for the marker.
In my case, I started out with a diagnosis of RA of the hips and spine as my back and hips hurt constantly, I had elevated inflammatory markers, persistent low grade fevers, and I had terrible morning stiffness, but I then developed uveitis and inflammation where my tendons attach to the bones in my shoulders so my diagnosis changed.

This might be helpful:
https://spondylitis.org/about-spondylitis/overview-of-spondyloarthritis/non-radiographic-axial-spondyloarthritis-nr-axspa/diagnosis/