Be observant. Signs of abuse may include frequently refilled prescriptions or hospital visits, track marks from needles on the skin and abnormal pupils (dilated/ constricted) or sclera (the whites of the eyes). Letting them know you are concerned and want to help rather than confronting them by accusation is the best approach to opening a discussion about the topic.
Change in habits. Change in habits, temperament hygiene can give a clue.