How can you tell if your breasts are the cause of pain in your back?

Exclude other causes: Very large, heavy breasts can definitely lead to upper back and neck pain. See your physician to look into any other causes such as musculoskeletal/spine problems. If they are excluded, then proper posture, excercise and a proper support bra can help. If not, then certainly breast reduction, a common, safe procedure for the right patient can dramatically reduce back and neck pains.