How would you know if si joint was unstable. Getting pain in left dimple area of back not there all day just when I move sometimes had it for 5 weeks.

Physical therapy. The pain may be coming from your si joint. If you bend sideways in the direction of the pain is at worse? The joint is probably not unstable, just irritated. Have your doctor send you to a physical therapist. They area great at pinpointing the source of pain and treating.
Testing. History, physical exam, xrays and testing. Often the most accurate method of diagnosing si joint dysfunction is by performing an injection that can numb the irritated area, thereby identifying the pain source.
Requires doctor eval. Diagnosing instability would require a doctors eval experienced with this problem. Pain in that area in your age is most often muscular, disk disease, or injury. Sorry i can't give you a specific answer.
Loose Ligaments . Often the si joint is irritated when the ligaments to the joint are loose. If physical therapy and strengthening do not improve your pain, consider seeing a spine specialist for an si joint injection or even prolotherapy to help strengthen the ligaments.