How would you determine if pain radiating down the back of the leg to above the knee, is sciatic vs. Referred pain from another site (hip, pelvis, etc)?

2nd opinion from DO. With your history of fractures, chronic back pain, OA knees B/L, back trauma,you could benefit greatly from seeing an osteopathic medical doctor, who specializes in neuromusculoskeletal medicine (NMM) to evaluate and treat you.They would be able to answer this question quickly,a nd accurately, as it may be a combination of both with your complex trauma history.There are DOs locally in ME,NH,RI &MA.
See your doctor. An office examination should point the way toward confirmatory tests and allow successful definitive diagnosis of condition causation.